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CORNELL

UNIVERSITY

LIBRARY

arV1831

Modern chromatics.

Cornell University Library

3 1924 031 167 889

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THE INTERNATIONAL SCIENTIFIC SERIES.

MODERN

CHEOMATIOS

WITH APPLICATIONS TO

AET AND II^DUSTRT.

OGDEN K^ EOOD,

PROFESSOR OF PHYSICS IN COLUMBIA COLLSaS.

WITH 130 ORIGINAL ILLUSTRATIONS.

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THIS VOLCME IS INSCRIBED,
AS A SMALL MAKK OF THE ATTACHMENT
^ AND ADMIBATION OF

THE AUTHOR.

PEEFAOE.

It was not my intention to write a preface to this
book, as I have usually found such compositions neither
instructive nor amusing. On presenting the manuscript
to my publishers, however, it was suggested that, al-
though prefaces are of no particxdar use to readers, yet
from a certain point of view they are not without value.

I accordingly beg leave to state that my object in this
M'ork has been to present, in a clear, logical, and if possi-
ble attractive form, the fundamental facts connected with
our perception of colour, so far as they are at present
known, or concern the general or artistic reader. For
the explanation of these facts, the theory of Thomas
Young, as modified and set forth by Helmholtz and
Maxwell, has been consistently adhered to. The whole
class of musical theories, as well as that of Field, have
been discarded, for reasons that are set forth in the text.

Turning now from the more purely scientific to the
sesthetic side of the subject, I will add that it has been
my endeavour also, to present in a simple and comprehen-
sible manner the underlying facts upon which the artistic
use of colour necessarily depends. The possession of these

vi PREFACE.

facts will not enable people to become artists ; but it may
to some extent prevent ordinary persons, critics, and even
painters, from talking and writing about colour in a loose,
inaccurate, and not always rational manner. More than
this is true : a real knowledge of elementary facts often
serves to warn students of the presence of difficulties
that are almost insurmountable, or, when they are already
in trouble, points out to them its probable nature ; in
short, a certain amount of rudimentary information
tends to save useless labour. Those persons, therefore,
who are really iuterested in this subject are iirged to
repeat for themselves the various experiments indicated
in the text.

In the execution of this work it was soon found that
many important gaps remained to be filled, and much
tune has been consumed in original researches and ex-
periments. The results have been briefly indicated in
the text ; the exact means employed in obtaining them
will be given hereafter in one of the scientific journals.

To the above I may perhaps be allowed to add, that
during the last twenty years I have enjoyed the great
privilege of familiar intercourse with artists, and duriag
that period have devoted a good deal of leisure time to
the practical study of drawing and painting.

O. K R.

OONTElfJ-TS.

CHAPTER I.

PACB

Transmission and Reflection op Light, 9

CHAPTER II.
Production of Colodk by Dispersion, . . , . 17

CHAPTER III.
Constants of Colour, . , .... 30

CHAPTER IV.
Production of Colour by Interference and Polabization, . 43

CHAPTER V.
Colours of Opalescent Media, .53

CHAPTER VI.
Production of Colour by Fluorescence and Phosphorescence, . 62

CHAPTER Vn.
Production of Colour by Absorption, . . 65

CHAPTER VIII.
Abnormal Perception of Colour and Colour-blindness, 92

CHAPTER IX.
Toung's Theory op Coloitr, 108

viii CONTENTS.

CHAPTER X.

PACE

Mixture of Colours, . . . . 124

CHAPTER XI.
Complementary Colours, . . . ■ . 161

CHAPTER XII.

Effects produced on Colour by a Change in Luminosity and by
MIXING it with White Light, 181

CHAPTER XIII.
Duration of the Impression on the Retina, .... 202

CHAPTER XIV.
Modes of arranging Colours in Systems, .... 209

CHAPTER XV.
Contrast, ... . . . . 235

CHAPTER XVI.

The Small Interval and Gradation, . . . 2'73

CHAPTER XVIL

Combinations of Colours in Pairs and Triads, . . .286

CHAPTER XVin.
Painting and Decoration 305

Note on two recent Theories op Colour, 324

Index, ... 326

MODERI CHROMATICS.

CHAPTER I.

THE REFLECTION AND TRANSMISSION OF LIGHT.

As long ago as 1795 it occurred to a German physicist
to subject tlie optic nerve of the living eye to the influence
of the newly discovered voltaic current. The result obtained
was curious : the operation did not cause pain, as might
have been expected, but a bright flash of light seemed to
pass before the eye. This remarkable experiment has since
that time been repeated in a great variety of ways, and
with the help of the more efiicient electric batteries of mod-
ern times; and not only has the original result of Pfaff been
obtained, but bright red, green, or violet, and other hues
have been noticed by a number of distinguished physicists.
If, instead of using the electrical current, mechanical force
be employed, that is, if pressure be exerted on the living
eye, the optic nerve is again stimulated, and a series of bril-
liant, changing, fantastic figures seem to pass before the
experimenter. All these appearances are distinctly visible
in a perfectly dark room, and prove that the sense of vision
can be excited without the presence of light, the essential
point being merely the stimulation of the optic nerve. In
the great majority of instances, however, the stimulation of
the optic nerve is brought about, directly or indii'ectly, by the

10 MODERN CHROMATICS.

aid of light ; and in the present work it is principally with
vision produced in this normal manner that we have to deal.
Back in the rear portion of the eye there is spread out a
delicate, highly complicated tissue, consisting of a wonder-
fully fine network woven of minute blood-vessels and
nerves, and interspersed with vast numbers of tiny atoms,
which under the microscope look like little rods and cones.
This is the retina ; its marvellous tissue is in some mysteri-
ous manner capable of being acted on by light, and it is
from its substance that those nerve-signals are transmitted
to the brain which awake in us the sensation of vision.
For the sake of brevity, the interior globular surface of the
retina is ordinarily called the seat of vision. An eye pro-
vided only with a retina would still have the capacity for
a certain kind of vision ; if plunged in a beam of red or
green light, for example, these colour-sensations would be
excited, and some idea might be formed of the intensity or
purity of the original hues. Some of the lower animals
seem to be endowed only with this rudimentary form of
vision ; thus it has lately been ascertained by Bert that
minute crustaceans are sensitive to the same colours of the
spectrum which affect the eye of man, and, as is the case
with him, the maximum effect is produced by the yellow
rays. With an eye constructed in this simple manner it
would, however, be impossible to distinguish the forms of ex-
ternal objects, and usually not even their colours. We have,
therefore, a set of lenses placed in front of the retina, and
so contrived as to cast upon it very delicate and perfect
pictures of objects toward which the eye is directed ; these
pictures are coloured and shaded, so as exactly to match the
objects from which they came, and it is by their action on
the retina that we see. These retinal pictures are, as it
were, mosaics, made up of an infinite number of points of
light ; they vanish with the objects producing them —
though, as we shall see, their effect lasts a little while after
they themselves have disappeared.

THE REFLECTION AND TRANSMISSION OF LIGHT. H

This leads us in the next place to ask, " What is light,
that agent which is able to produce effects which to a
thoughtful mind must always remain wonderful ? " A per-
fectly true answer to this question is, that light is some-
thing which comes from the luminous body to us ; in the
act of vision we are essentially passive, and not engaged in
shooting out toward the object long, delicate feelers, as was
supposed by the ancients. This something was considered
by Sir Isaac Newton to consist of fine atoms, too fine al-
most to think of, but moving at the rate of 186,000 miles in
a second. According to the undulatory theory, however,
light consists not of matter shot toward us, but of undula-
tions or waves, which reach our eyes somewhat in the same
way as the waves of water beat on a rocky coast.

The atoms, then, which compose a candle flame are
themselves in vibration, and, communicating this vibratory
movement to other particles with which they are in con-
tact, generate waves, which travel out in all directions, like
the circular waves from a stone dropped into quiet water ;
these waves break finally upon the surface of the retina,
and cause in some unexplained way the sensation of sight —
we see the candle flame. Substances which are not seK-
luminous cannot be seen directly or without help ; to ob-
tain vision of them it is necessary that a self-luminous body
also should be present. The candle flame pours out its
flood of tiny waves on the objects in the room ; in the act
of striking on them some of the waves are destroyed, but
others rebound and reach the eye, having suffered certain
changes of which we shall speak hereafter.

This rebound of the wave we call reflection ; all bodies
in the room reflect some of the candle light. Surfaces which
are polished alter the direction of the waves of light falling
on them, but they do not to any great extent scatter them
irregularly, or in all directions. It hence follows that pol-
ished surfaces, when they reflect light, present appearances

12 MODERN CHROMATICS.

totally unlike those furnislied by surfaces which, though
smooth, are yet destitute of polish ; the former are apt to
reflect very much or very little light, according to their
positions, but this is not true to the same extent with un-
polished surfaces. The power which difllerent substances
have under various circumstances to reflect light is not
without interest for us ; we shall see hereafter that this is
a means often employed by nature in modifying colour.

As a general thing polished metallic surfaces are the
best reflectors of light, and may for the most part be con-
sidered by the artist as reflecting all the light falling on
them. Polished silver actually does reflect ninety-two per
cent, of the light falling perpendicularly on it ; and though
the percentages reflected by steel and other metals are
smaller, yet the difference is not ordinarily and easUy dis-
tinguished by an untrained eye.

The case is somewhat different with smooth water : if
light falls on it, making a small angle with its surface, the
amount reflected is as large as that from a metallic surface ;
while, if the light falls perpendicularly on it, less than four
per cent, is reflected. Thus with a clear blue sky and
smooth water we find that distant portions of its surface
appear very bright, while those at the feet of the observer
are of an almost unbelievable dark-blue tint. In this par-
ticular instance, the difference between the brightness of
near and distant portions of the water is still further exag-
gerated by the circumstance that the sky overhead is less
luminous than that near the horizon ; and the distant por-
tions of the sheet of water reflect light which comes from
the horizon, the nearer portions that which has its origin
overhead. The reflecting power of water is constantly
used by artists as a most admirable means of duplicating in
a picture a chromatic composition, and easily affords an op-
portunity, by slight disturbances of its surface, ior the
introduction of variations on the original chromatic design.

It may here be remarked that in actual landscapes con-

THE REFLECTION AND TRANSMISSION OF LIGHT. 13

taining surfaces of still water, it ordinarily happens that
the reflected pictures are not exactly identical with those
which are seen directly, and the difference may often be
considerable. For example, it may easily be the case that
an object beyond the water, and situated at some distance
from it, is not seen in the reflected picture at all, light from
it either not reaching the water, or reaching the water and
not being reflected to the eye of the observer.

Polished surfaces, as we have seen, reflect light not only
in large quantity, but they as it were press the light well
together in rather sharply defined masses ; with unpolished
surfaces the case is entirely different, the light which falls
on them being scattered in all directions. Hence, where-
ever the eye is placed, it receives some of this light, and a
change of position produces far less effect on the quantity
received than is the case with light reflected from polished
surfaces. Owing to their power of scattering light in all
directions, rough surfaces, however situated, never send
very intense light to the eye.

If a surface of white linen drapery be illuminated by a
dozen different sources, it will reflect to the eye a sample of
each kind of light, and what we call its hue will be made
up of as many constituents. When we remember that all
the different objects in a room reflect some, and usually
coloured light, we see that the final tint of our piece of linen
drapery depends not only on the circumstance that its natu-
ral colour is white, but also on the presence and proximity of
curtains, books, chaii's, and a great variety of objects ; the
final colour will hence not be exactly white, but some delicate,
indescribable hue, difficult of imitation except by practiced
artists. With objects which are naturally coloured, or which
show colour when placed in white light, the case becomes
still more complicated. Let us suppose that our drapery
when placed in pure white light appears red ; its hue will

14 MODERN CHROMATICS.

still be modified by the light it receives from objects ia the
room : for example, if it receives some green light from
objects of this colour placed in its neighbourhood, the red
hue will incline toward orange; if the added portion of
light be yellow, the tendency to orange will be still more
marked ; on the other hand, light received from blue or
violet surfaces will cause the red to pass into crimson or
even purple. The grandest illustrations of these changes
we find in those cases where objects are illuminated simul-
taneously by the yellow rays of the sun and the blue light
of the clear sky : here, by this cause alone, the natural
colours of objects are modified to a wonderful extent, and
effects of magical beauty produced, which by their intricacy
almost defy analysis. The nature of these changes will be
considered in a subsequent chapter, after the principles upon
which they depend have been examined.

Finally, it may not be altogether out of place to add
that the majority of paintings and chromatic designs are
seen by the aid of light which they reflect in a diflfused way
to the eye of the observer ; transparencies, designs in stained
or painted glass, etc., are, on the other hand, seen by light
which passes entirely through their substance before reach-
ing the eye. Corresponding to this we find that by far the
larger proportion of natural objects act upon om- visual
organs by means of reflected light, while a few only are
seen by a mixture of reflected and transmitted light. It
hence follows that Nature and the painter actually employ,
in the end, exactly the same means in acting on the eye of
the beholder. This point, seemingly so trite, is touched
upon, as an idea seems to prevail in the minds of many per-
sons that Nature paints always with light, while the artist
is limited to pigments : in point of fact, both paint with
light, though, as we shall hereafter see, the total amount at
the disposal of the painter is quite limited.

THE REFLECTION AND TRANSMISSION OF LIGHT. 15

In concluding this matter of reflection, we may perhaps
be allowed to add that the term reflection is quite frequent-
ly confused with shadow — the reflected image of trees on
the edge of quiet water being often spoken-of as the shadows
of trees on the water. The two cases are of course essen-
tially different, a genuine, well-defined shadow on water
scarcely occurring except in cases of turbidity.

We have seen that all bodies reflect some of the light
falling on them ; it is equally true that they transmit a
certain portion. A plate of very pure glass, or a thin layer
of pure water, will transmit all the light falling on it, ex-
cept that which is reflected ; they transmit it unaltered in
tint, and we say they are perfectly transparent and colour-
less substances. Here we have one of the extremes ; the
other may be found in some of the metals, such as gold or
silver : it is only when they are reduced to very thin leaves
that they transmit any light at all. Gold leaf allows a lit-
tle light to pass through its substance, and tinges it bluish-
green. Almost all other bodies may be ranged between
these two examples ; none can be considered absolutely
transparent, none perfectly opaque. And this is true not
only in a strictly philosophical sense, but also in one that
has an especial bearing on our subject. The great mass of
objects with which we come in daily contact allow light to
penetrate a little way into their substance, and then, turn-
ing it back, reflect it outward in all directions. In this
sense all bodies have a certain amount of transparency.
The light which thus, as it were, just dips into their sub-
stance, has by this operation a change impressed on it ; it
usually comes out more or less coloured. It hence follows
that, in most cases, two masses of light reach the eye : one,
which has been superficially reflected with unchanged colour;
and another, which, being reflected only after penetration,
is modified in tint. Many beautiful effects of translucency
are due to these and strictly analogous causes ; the play

16 MODERN CHROMATICS.

of colour on the surfaces of waves is made up largely of
these two elements ; and in a more subdued way we find
them also producing the less marked translucency of foliage
or of flesh.

One of the resources just mentioned the painter never
employs : the light which is more or less regularly reflected
from the outermost surface, he endeavours to prevent from
reaching the eye of the beholder, except in minute quanti-
ty, his reliance being always on the light which is reflected
in an irregular and diffused way, and which has for the
most part penetrated, first, some little distance into his
pigments.

The glass-stainer and glass-painter make use of the
principle of the direct transmission of light for the display
of their designs. Now, as painted or stained glass trans-
mits enormously more light than pigments reflect in a prop-
erly lighted room, it follows that the worker on glass has at
his disposal a much more extensive scale of light and shade
than the painter in oils or water-colours. Owing to this
fact it is possible to produce on glass, paintings which, in
range of illumination, almost rival Nature. The intensity
and purity of the tints which can thus be produced by
direct transmission are far in advance of what can be ob-
tained by the method of reflection, and enable the designer
on glass successfully to employ combinations of colour
which, robbed of their brightness and intensity by being
executed in oils or fresco, would no longer be tolerable.

CHAPTER II.

PRODUCTION OF COLOUR BY DISPERSION.

In the previous chapter we have seen that the sensation
of sight is produced by the action of very minute waves on
th& nervous substance of the retina ; that is to say, by the
aid of purely mechanical movements of a definite character.
When these waves have a length of about -5-5^57 •'^ ^^ inch,
they produce the sensation which we call red — we see red
light ; if they are shortened to xr^TTir of an inch, their ac-
tion on us changes, they call up in us a different sensation
— we say the light is coloured orange ; and as the lengths
of the waves are continually shortened, the sensation passes
into yellow, green, blue, and viplet. From this it is evident
that colour is something which has no existence outside and
apart from ourselves ; outside of ourselves there are merely
mechanical movements, and we can easily imagine beings
so constructed that the waves of light would never produce
in them the sensation of colour at all, but that of heat.

The colour-sensations just mentioned are not the only
ones capable of being produced by the gradual diminution
of the wave-length : between the red and orange we find
every variety of orange-red and red-orange hue ; the or-
ange, again, changes by a vast number of insensible steps
into yellow, and so of all the other tints. Types of all
colours possible, except the purples, could be produced by
this method. The colours generated in this way would not
only pass by the gentlest gradations into each other, form-
ing a long scries of blending hues, but they would also be

MODERN CHROMATICS.

perfectly pure, and, if the light was bright, very intense.
The advantage of providing, in the beginning of our colour
studies, a set of tints possessiug these precious qualities, is
evident without much argument.

Now, white light consists of a mixture of waves pos-
sessing every desirable degree of length, and it is only ne-
cessary to select some instrument which is able to sort out
for us the different kinds of light, and neatly arrange them
side by side in an orderly series. Fortunately for us, we
find in the glass prism a simple and inexpensive apparatus
which is able to effect the desired analysis. "We may, Lf we
are willing to take a little trouble, arrange matters so as to

Fi&. 1. — ^Prismatic Spectrum.

repeat the famous experiment made by Newton many years
ago : viz., admit a small beam of sunlight into a darkened '
room, and allow it to fall on the prism, as indicated in Fig.
1. We shall notice, by observing the illuminated path of
the sunbeam, that the prism bends it considerably out of
its course ; ajid, on tracing up this deflected portion, we
shall find it no longer white, but changed into a long streak
of pure and beautiful colours, which blend into each other
by gentle gradations. If this streak of coloured light be
received on a white wall, or, better, on a large sheet of
white cardboard, the following changes in the colours can

PRODUCTION OF COLOtJE BY DISPERSION. 19

be noticed : It commences at one end with a dark-crimson
hue, which gradually brightens as we advance along its
length, changing at the same time into scarlet ; this runs
into orange, the orange becomes more yellowish, and con-
trives to convert itself into a yellowish-green without pass-
ing noticeably into yellow, so that at first sight yellow does
not seem to be present. The orange-yellow and greenish-
yellow spaces are brighter than any of the others, but the
rise in luminosity is so gradual that the difference is not
striking, unless we compare these two colours with those at
a considerable distance from them. As we pass on, the ten-
dency to green becomes more decided, until finally a full
green hue is reached. This colour is still pretty bright,

I^a. 2.— Mode of isolating a Single Colour of the Prismatic Spectrum.

and not inferior to the red in intensity ; by degrees it
changes into a greenish-blue, which will not at first attract
the attention ; next follows a full blue, not nearly so bright
as the green, nor so striking ; this blue changes slowly into
a violet of but little brightness, which completes the series.
If we wish to isolate and examine these tints separately,
we can again follow the example of Newton, by making a
small, narrow aperture in our cardboard, and use it then as
a screen to intercept all except the desired tint, as is indi-
cated in Fig. 2. In this manner we can examine separate

20 MODERN CHROMATICS.

portions of our spectrum more independently, and escape
from the overpowering influence of some of the more in-
tense tints. Under these circumstances the greenish-hlue
becomes quite marked, and the blue is able to assert itself
to a greater degree ; but the yellow will not be greatly
helped, for in fact it is confined to a very narrow region,
and it is only by greatly magnifying the spectrum that we
can obtain a satisfactory demonstration of its existence.

These experiments, though very beautiful, are quite
rough ; every two minutes the beam of sunlight strays
away from the prism and needs again to be directed toward
it ; and besides that, the colours blend into each other in
such a subtile, puzzling way, that, without a scale or land-
mark of some kind to separate them, it seems hopeless to
undertake any exact experiments. In this diflSculty it is to
the spectroscope that we must turn for aid ; it was certainly
not originally contrived for such purposes as these, but
nevertheless is just what we need. It is not necessary to.
stop to describe the instrument, as this has been done by
Professor Lommel in another volume of this series ; it is
enough for us that it is a convenient instrument for sorting
out the different kinds of light which fall on it, according
to their wave-length, and that it performs this work far
more accurately than a prism used according to Newton's
plan. Just at this point we can take advantage of a sin-
gular discovery made by Fraunhofer, and independently
to some extent by Dr. Wollaston, early in the present cen-
tury. These physicists found that when the coloured band
of light just described is produced by a spectroscope, or
by apparatus equivalent to jone, the band is really not con-
tinuous, but is cut up crosswise into a great many small
spaces. The dividing lines are called the fixed lines of the
solar spectrum. Almost their sole interest for us is in the
fact that they serve as admirable landmarks to guide us
through the vague tracts of ill-defined colour. Fig. 3 shows
the positions of some of the more important fixed lines of

PRODUCTION OF COLOmi BY DISPERSION.

* It will be noticed that the term indigo,
originally Introduced by Newton, haa been
entirely rejected in thia work, and ultrama-
rine aubatituted for it. Bezold auggested
thia change aome time ago, baaing hia ob-
jection to indigo on ita dingineaa ; the au-
thor, however, finds a much more fatal ob-
jection in the fact that indigo in solution,
and as a pigment, ia a somewhat greenish-
blue, being really identical with Prussian-
blue in colour, only far blacker. In the
dry atate thia tendency to greenneaa ia neu-
tralized by the reddish tinge which the aub-
Btanoe sometimes assumes : it was probably
used by Newton in the dry state. A mix-
ture of aix parta of artificial ultramarine-
bluo, two parta white, and ninety-two parts
black, when mingled according to the meth-
od of Maxwell's disks, furnishes a colour
quite like that of commercial indigo in the
dry atate.

the spectrum. The figure is based on measurements made
by the author on a flint-glass prism,
with aid of a large spectroscope,
or rather spectrometer, admirably
constructed by Wm. Grunow, of
New York. At the same time a
series of observations was made
on the extent of the coloured
spaces in the spectrum ; these are
indicated in the figure, and ac-
curately given in one of the ta-
bles that follow.* Let us sup-
pose that the spectrum from A to
H includes 1,000 parts ; then the
following table indicates the po-
sitions of the fixed lines :

* Eed-oran^e.

Orauge.

Orange-yellow.*

Yellow.

Green-yellow
> and
Yellow-green.

Green and
Blue-green.

» Cyan-blue.

V Blue and
/ Blue-violet.

> Violet

Fio. 8.— Fixed Lines and Coloured
Spaces of FriBuiatic Spectrum.

MODERN CHEOMATICS.

Fixed Likes of the Pkismatic Spectrum.

4005

74-02

C... 112-'71

D 220-31

A.
a.
B.

E 363-11

6 389-85

F 493-22

G VSS-BS

n 1000-00

The next table gives the positions of the coloured spaces
in this spectrum, according to the observations of the author :

Oolouked Spaces in the Pkismatic SPEOTEtiM.

Red begins at 0

Red ends, orange-red begins at 149

Orange-red ends, orange begins at 194

Orange ends, orange-yellow begins at 210

Orange-yellow ends, yellow begins at 230

Yellow ends, greenish-yellow begins at 240

Tellow-greeB ends, green begins at 344

Blue-green ends, cyan-blue begins at 447

Cyan-blue ends, blue begins at 495

Violet-blue ends, violet begins at 806

Violet ends at ■ 1,000

The space out beyond 0 is occupied by a very dark red,
which has a brown or chocolate colour, and outside of the
violet beyond 1,000 is. a faint greyish colour, -which has been
called lavender.

The third table shows the spaces occupied in the pris-
matic spectrum by the several colours :

Red , 149

Orange-red 45

Orange X6

Orange-yellow , _ _ 20

Yellow IQ

Greenish-yellow and yellowish-green 104

Green and blue-green IO3

Cyan-blue 48

Blue and blue-viol<et 311

"Violet 194

1,000

PRODUCTION OF COLOUR BY DISPERSION. 23

In making these observations, matters were arranged so
that only a narrow slice of the spectrum presented itself to
the observer ; thus its hues could be studied in an isolated
condition, and the misleading effects of contrast avoided.
The figures given in the two latter tables are the mean of
from fifteen to twenty observations. The hues of the spec-
tral colours change very considerably with their luminosity ;
hence for these experiments an illumination was selected
such that it was only comfortably bright in the most lumi-
nous portions of the spectrum, and this arrangement re-
tained as well as possible afterward.

The colours as seen in the spectroscope really succeed
each other in the order of their wave-lengths, the red hav-
ing the greatest wave-length, the violet the least. But the
glass prism does this work in a way which is open to criti-
cism ; it crowds together some portions of the series of tints
more than is demanded by their difference in wave-lengths ;
other portions it expands, assigning to them more room
than they have a right to claim. Thjis the red, orange,
and yellow spaces are cramped together, while the blue and
violet tracts stretch out interminably. Taking all this into
consideration, it may be worth while to go one step further,
and, without abandoning the use of the spectroscope, re-
place its prism by a diffraction grating, or plate of glass
ruled with very fine, parallel, equidistant lines, such as have
been made by the celebrated Nobert, and lately of still
superior perfection by Rutherfurd. In Lommel's work,
previously referred to, the mode in which a plate of this
kind produces colour is explained ; at present it is enough to
know that the general appearance of the spectacle will be
unchanged ; the same series of colours, the same fixed lines,
will again be recognized ; but in this new spectrum all the
tints will be arranged in an equable manner with reference
to wave-length. According to this new allotment of spaces,
the yellow will occupy about the centre of the spectrum,

24 MODERN CHROMATICS.

tlie red and different kinds of orange taking up more room
than formerly; the dimensions of the blue and violet will
he greatly reduced.

Let us suppose, as before, that the spectrum from A to
H includes 1,000 parts ; then the following table, which is
calculated from the observations of Angstrom, will indi-
cate the positions of the principal fixed lines :

Fixed Lines in the Nokmal Spectrum.

A 0

a llS-n

B 201-61

C 285-05

D 468-38

E 688-92

b eei-M

F '?49-24

G 90207

H ....1000-00

The next table gives the positions of the coloured spaces
in the normal spectrum, according to the observations of
the author :

Coloured Spaces in the Normal Spectrum.

Red begins at 0

Pure red ends, orange-red begins at 330

Orange-red ends, orange begins at 434

Orange fends, orange-yellow begins at 459

Orange-yeUow ends, yellow begins at 485

Tellow ends, greenish-yellow begins at 498

Yellow-green ends, full green begins at ; . . . 596

Full green ends, blue-green begins at 682

Blue-green ends, cyan-blue begins at 698

Cyan-blue ends, blue begins at 749

Blue ends, violet-blue begins at 823

Blue-violet ends, pure violet begins at 940

The following table exhibits the spaces occupied by the
several colours in the normal spectrum :

Pure red 830

Orange-red 104

Orange 25

Orange-yellow 26

PRODUCTION OF COLOUR BY DISPERSION.

Yellow IS

Greenish-yellow and yellow-green . 9"?

Full green 87

Blue-green 16

Cyan-blue 51

Blue 74

Violet-blue and blue-violet 117

Pure violet 60

1,000

Fig. 4 shows the normal
spectrum with fixed lines and
coloured spaces, corresponding
to the tables just given.

If these tables are compared
with those obtained by the aid
of a prism of glass, it will be
seen that the fixed lines and
coloured spaces are arranged
somewhat differently; the
main cause of this difference
has already been pointed out.
When, however, we compare
the spacing of the colours in
the two spectra, it is also to be
remembered that it is affected
by another circumstance, viz.,
the distribution of the lumi-
nosity in the two spectra does
not agree, and this influences,
as will be shown in Chapter
XII., the appearance of the
colours themselves ; very lu-
minous red, for example, as-
suming an orange hue, very
dark blue tending to appear
violet, etc. The normal spectrum employed by the autho

. Eed.

. Orange-red.

■ Orange.
. Orange-yellow.

■ Yellow.

Greenish-yellow.

■ and
Yellowish-green.

■ Green.

• Blue-green.
. Cyan-blue.

. Blue.

Violet-blue.

Violet.

Fig. 4. — Fixed Lines and Coloured
Spaces of Normal Spectrum.

26 MODERN CHROMATICS.

was obtained by using a superb plate for which he was
indebted to Mr. Rutherfurd. The plate contained nearly
19,000 lines to the English inch, and was silvered on the
back, so that the colours were as bright as those from a
glass prism. The spectrum selected for use was nearly six
times as long as that furnished by the glass prism — a cir-
cumstance, of course, that favoured accurate observation.

The tables that have just been given enable us very
easily to calculate the lengths of the waves of light, cor-
responding to the centres of the coloured spaces in the nor-
mal spectrum. It is only necessary to ascertain the number
corresponding, for example, to the centre of the red space,
then to multiply it by 3'653, and to subtract the product
from 7,603 : the result will be the wave-length correspond-
ing to that part of the normal spectrum, expressed in ten-
millionths of a millimetre. The following table contains
the wave-lengths corresponding to the centres of the col-
oured spaces :

I U.OUO,(Tu u MM.

Centre of red V,000

" orange-red , 6,208

" orange , 5,972

" orange-yellow 5,8'79

" yellow '. 5,808

" full green 5,271

" blue-green 5,082

" cyan-blue 4,960

" blue 4,732

" violet-blue 4 383

" pure violet 4 059

The results here given differ somewhat from those obtained
by Listing in 1867 ; the differences are partly due to the
terms employed ; the author, for example, dividing up into
orange-red, orange, and orange-yellow, a space which is
called by Listing simply orange. According to the author

PRODUCTION OF COLOUR BY DISPERSION. 27

cyan-blue falls on the red side of the line F ; it is placed
by Listing, however, on the violet side of this line. Other
less important differences might be mentioned ; but, as a dis-
cussion of them would be out of place in a work like the
present, the curious reader is referred for further informa-
tion to Listing's paper.*

A little study of the normal spectrum, Fig. 4, will enable
us to answer some interesting questions. We have already
seen that change in colour is always accompanied by change
in the length of the waves of light producing it ; hence if we
begin at one end of our normal spectrum where the colour
is red, and the length of the waves equal to 7,603 ten-
millionths of a millimetre, as we diminish this length, we
expect to see a corresponding change in the colour of the
light : small changes we anticipate will produce small effects
on the colour, large changes greater effects.

Now, the question arises whether equal changes of wave-
length actually are accompanied by equal alterations of hue
in all parts of the spectrum. To take an example : in pass-
ing from the orange-yellow, through the pure yellow and
greenish-yellow well into the yellow-green region, we find
it necessary to shorten our wave-length about 400 of our
units ; now will an equal curtailment in other regions of the
spectrum carry us through as many changes of hue ? The
answer to this is not exactly what we might expect. In a
great part of the red region a change of this kind produces
only slight effects, the red inclining a little more or less to
orange, and the same is true of the blue and violet spaces,
the hue leaning only a little toward the blue or violet side,
as the case may be. Hence it seems that the eye is far
more sensitive to changes of wave-length in the middle
regions of the spectrum than at either extremity. This
circumstance, to say the least, is curious ; but, what is more
to our purpose, it is a powerful argument against any theory

* Poggendorfifs " Anualen," cxxxi., p. 564.

MODERN CHROMATICS.

of colour whicli is founded on supposed analogies with
music. But more of this hereafter.

In the prismatic spectrum and in our normal spectrum
we found no representative of purple, or purplish tints.
This sensation can not be produced by one set of waves
alone, whatever their length may be ; it needs the joint
action of the red and violet waves, or the red and blue. All
other possible tints and hues find their type in some portion
of the spectrum, and, as will be shown in the next chapter,
this applies ' jast as well to the whole range of browns and
greys, as to colonic like vermilion and ultramarine.

We have seen that the mixture of long and short waves
which compose white light can be analyzed by a prism into
its original constituents : the long waves produce on us the

Fig. 5.— Eecomposition of White Light.

sensation that we call red, and, as we allow shorter and
shorter waves to act on the eye, we experience the sensa-
tions known as orange, yellow, green, blue, and violet.
"When, on the other hand, we combine or mix together these
different kinds of light, we reproduce white light. There
are a great many different ways of effecting this recom-

PEODTJCTION OF COLOUR BY DISPERSION. 29

position ; one of the most beautiful was contrived several
years ago by Professor Eli Blake. Tlie spectrum is re-
ceived on a strip of ordinary looking-glass, which is gently
bent by the hands of the experimenter till it becomes some-
what curved ; it then acts like a concave mirror, and can be
made to concentrate all the coloured rays on a distant sheet
of paper, as shown in Fig. 5. The spot where all the col-
oured rays are united or mixed appears pure white.

CHAPTER in.

THE CONSTANTS OF COLOUR.

The tints produced by Nature and art are so manifold,
often so vague and indefinite, so affected by their environ-
ment, or by the illumination under which they are seen,
that at first it might well appear as though nothing about
them were constant ; as though they had no fixed proper-
ties which could be used in reducing them to order, and in
arranging in a simple but vast series the immense multitude
of which they consist.

Let us examine the matter more closely. We have seen
that when a single set of waves acts on the eye a colour-
sensation is produced, which is perfectly well defined, and
which can be indicated with precision by referring it to
some portion of the spectrum. We have also found that
when waves of light, having all possible lengths, act on the
eye simultaneously, the sensation of white is produced.
Let us suppose that by the first method a definite colour-
sensation is generated, and afterward, by the second meth-
od, the sensation of white is added to it : white light is
added to or mixed with coloured light. This mixture may
be accomplished by throwing the solar spectrum on a large
sheet of white paper, and then casting on the same sheet of
paper the white light which is reflected from a silvered
mirror, or from an unsilvered plate of glass. Fig. 6 shows
the arrangement. By moving the mirror M, Fig. 6, the
white band of light may be made to travel slowly over the
whole spectrum, and thus furnish a series of mixtures of

THE CONSTANTS OF COLOUR. 31

white light with the various prismatic hues. The general
effect of this proceeding will be to diminish the action of
the coloured light ; the mixtui'e will indeed present to the
eye more light, but it will be paler ; the colour-element will
begin to be pushed into the background. Conversely, if
we now should subject our mixture of white and coloured
light to analysis by a second prism, we should infallibly
detect the presence of the white as well as of the coloured
light ; or, if no white light were present, that would also

Fio. 6.— Mode of mixing White LigM with the Colours of the Spectrum.

be equally apparent. Taking all this into consideration, it
is evident that, when a particular colour is presented to us,
we can affirm that it is perfectly pure ; viz., entirely free
from white light, or that it contains mingled with it a
larger or smaller proportion of this foreign element. This
furnishes us with our first clue toward a classification of
colours : our pure standard colours are to be those found in
the spectrum ; the coloured light coming from the surfaces
of natural objects, or from painted surfaces, we must com-
pare with the hues of the. spectrum. If this is done, in al-
most every case the presence of more or less white light
will be detected ; in the great majority of instances its

33 MODEEN CHROMATICS.

preponderance over the coloured light will be found quite
marked. To illustrate by an example : if -white paper be
painted with vermilion, and compared with the solar spec-
trum, it will be noticed that it corresponds in general hue
with a certain portion of the red space ; but- the two coloui-s
never match perfectly, that from the paper always appear-
ing too pal.e. K, now, white light be added to the pure
spectral- tint, by reflecting a small amount of it from the
mirror (Fig. 6), it will become possible to match the two
colours ; and, if we know how much white light has been
added, we can afterward say that the light reflected from
the vermilion consists, for example, of eighty per cent, of
red light from such a region of the spectrum, mixed with
twenty per cent, of white light. If we make the experi-
ment with a surface painted with " emerald green," we
shall obtain about the same result, while we shall find that
artificial ultramarine-blue, reflects about twenty-five per
cent, of white light. In all of these cases the total amount
of light reflected by the coloured paper is of course taken
as 100, and the results here given are to be regarded only
as approximations. In every case some white light is sure
to be present ; its effect is to soften the colour and reduce its
action on the eye ; when the proportion of white is very
large, only a faint reminiscence of the original hue remains :
we say the tint is greenish-grey, bluish-grey, or reddish-
grey. If one part of red light is mixed with sixteen parts
of white light, the mixture appears of a pale pinkish hue.
The specific effects produced by the mixture of white with
coloured light will be considered in Chapter XII. ; it is
enough for us at present to have obtained an idea of one of
the constants of colour, viz., its purity. The same word, it
may- be observed, is often used by artists in an entirely dif-
ferent sense : they will remark of a painting that it is no-
ticeable for the purity of its colour, meaning only that the
tints in it have no tendency to look dull or dirty, but not at
all implying the absence of white or grey light.

THE CONSTANTS OF COLOUR. 33

Next let us suppose that in our study of these matters
we have presented to us for examination two coloured sur-
faces, which we find reflect in hoth cases eight tenths red
light and two tenths white light. In spite of this, the tints
may not match, one £i£ them being much brighter than the
other ; containing, perhaps, twice as much red light and
twice as much white light ; having, in other words, twice
as great brightness or luminosity. The only mode of caus-
ing the tints to match will be to expose the darker-coloured
surface to a stronger light, or the brighter surface to one
that is feebler. It is evident, then, that brightness or lumi-
nosity is one of the properties by which we can define col-
our ; it is our second colour-constant. This word luminos-
ity is also often used by artists in an entirely different sense,
they calling colour in a painting luminous simply because it
recalls to the mind the impression of light, not because it
actually reflects much light to the eye. The term " bi-ight
colour " is sometimes used in a somewhat analogous sense
by them, but the ideas are so totally different that there is
little risk of confusion.

The determination of the second constant is practicable
in some cases ; it presents itself always in the shape of a
difficult photometric problem. The relative brightness of
the colours of the solar spectrum is one of the most inter-
esting of these problems, as its solution would serve to give
some idea of the relative brightness of the colours which,
taken together, constitute white light. Quite recently a
set of measurements was made in different regions of the
spectrum by Vieror(Jt, who referred the points measured to
the fixed lines, as is usual in such studies.* Reducing his
designations of the different regions of the spectrum to
those of our spectral chart, which includes 1,000 parts from
A to H (see previous chapter), and supplying the colours
from the observations of the present writer, we obtain the
following table :

* 0. Vierordt, Poggendorff'a " Annalen," Band cxxxvii., S. 200.

MODERN CHROMATICS.

Table showing the Luminosity of Different Regions op the Prismatic

SrEOTEUM.

Position.

Luminosity.

Colour.

From 40-5 to

Dark red.

" 104-5 "

112-71

493

Pure red.

" 112-'n "

138-6

1,100

Red.

" 158-5 "

168-5

2,773.

Orange-red.

" 189

220-31

6,985

Orange and orange-yellow.

" 220-31 "

231-5

7,891

Orange-yellow.

" 231-5 "

363-11

3,033

Greenish-yellow, yellow-green, and green.

" 389-85 "

493-22

1,100

Blue-green and cyan-blue.

" 493-22 "

558-5

493

Blue.

" 623-5 "

689-5

90-6

TJltramarine (artificial).

" 753-58 "

825-5

36-9

Blue-violet.

" 896-5 "

956

13-1

Violet.

The author finds that -with the aid of rotating disks the
second, constant can often be determined.* Let us suppose
that we -wish to determine the luminosity of paper painted
■with vermilion : a circular disk, ahout six inches in diame-
ter, is cut from the paper and placed on a rotation apparatus,
as indicated in Fig. 7. On the same axis is fastened a double
disk of black and of -white paper, so arranged that the pro-
portions of the black and -white can be varied at -will.f When
the -whole is set in rapid rotation, the colour of the vermilion
paper -will of course not be altered, but the black and white
-will blend into a grey. This grey can be altered in its
brightness, till it seems about as luminous as the red (Fig.
8). If we find, for example, that -with the disk three quar-
ters black and one quarter white an equality appears to be
established, we conclude that the luminosity of our red sur-
face is twenty-five per cent, of that of the white paper. This

* " American Journal of Science and Arts," February, 1878.
■f See Maxwell's " Disks," chapter x.

THE CONSTANTS OP COLOUR.

is of course based on the assumption that the black paper
reflects no light ; it actually does reflect from two to six
per cent., the reflecting power of white paper being put at
100. The black disk used by the author reflected 5-2 per

Fig. 7.— Coloured Disk, with Small Blaok-and-White I

Disk.-

White Disk in Kotation.

cent, of white light ; to meet this a correction was intro-
duced, and a series of measurements made, some of the more
important of which are given in the following table :

Luminosity.

White paper 100

Vermilion (English)* 25'7

Pale chrome-yellow f 803

Pale emerald-green* 48'6

Cobalt-blue t 36-4

Ultramarine J: 1-6

These results were afterward tested by the use of a set
of disks, the colours of which were complementary to those
mentioned in the table, and these additional experiments
and calculations showed that the original measurements
differed but little from the truth. This agreement proved
also the correctness of Grassmann's assumption, that the total

* la thick paste, f Washed on as a water-colour. % Artificial, as a
paste.

36 MODERN CHROMATICS.

intensity of the mixture of masses of differently coloured
'ligtt is equal to the sum of the intensities of the separate
components.

But to resume our search for colour-constants. We may
meet with two portions of coloured light having the same
degree of purity and the same apparent brightness, which
nevertheless appear to the eye totally different : one may
excite the sensation of blue, the other that of red ; we say
the hues are entirely different. The hue of the colour is,
then, our third and last constant, or, as the physicist would
say, the degree of refrangibility, or the wave-length of the
light. In the preceding chapter it has been shown that the
spectrum offers all possible hues, except the purples, weU
arranged in an orderly series, and the purples themselves
can be produced with some trouble, by causing the blue or
violet of the spectrum to mingle in certain proportions with
the red.

Fis. 9.— Eye-piece with Dalton's Scale.

For the determination of the hue, an ordinary one-prism
spectroscope can be used ; it is only necessary to add a little
contrivance which enables the observer to isolate at will any

THE CONSTANTS OF COLOITK

small portion of tlie spectrum. TMs object is easily at-
tained by introducing into tbe eye-piece of the instrument

Fia. 10. — BufherfUrd's Automatlo Spectroscope.

a- diaphragm perforated by a very narrow slit (see Fig. 9).
The observer then sees in the upper part of the field of view
the selected spectral colour ; in the lower part of the field
the scale is visible, and with its aid the precise position of
the prismatic hue can be determined. Instead of using a

MODERN OHKOMATICS.

scale divided into equal parts, it is often advantageous to
employ the plan suggested by Dr. J. C. Dalton, and used
by him for determining the position of certain absorption
bands. Dr. Dalton employs as a scale a minute photograph
which shows the positions of the fixed lines, and divides up
the spaces between them conveniently. Fig. 9 exhibits the
appearance of the field of view and the scale. For more
accurate work Rutherfurd's automatic six-prism spectro-
scope can be employed (see Fig. 10*). A diffraction grat-
ing can also be used — in those cases where the student of
colour is so fortunate as to possess one.f With a very per-
fect grating of this kind, for which the author was indebted
to Mr. Rutherfurd, the third constant was determined for a
number of coloured disks. The following table gives their
positions in a normal spectrum haviug from A to H 1,000
parts ; the corresponding wave-lengths are also given :

Name of the Colour.

Vermilion (English)

Red lead

Pale chrome-yellow

Emerald-green

Prussian-blue

Cobalt-blue

Ultramarine (genuine)

Ultramarine (artificial)

Same, washed^ With Hoffmann's Tiolet
B. B .\

Position in tlie
Normal Spectram.

387
422
4S8
648
740
770
785
857

916

Wave-length .
11 Tsoiresire ™™-

6,290
6,061
^,820
5,234
4,899
4,790
4,735
4,472

4,257

We have seen that the first colour-constant has reference
to the purity of the colour, or indicates the relative amount
of white light mixed with it. This constant is in all cases

* Fig. 10 is a facsimile of Rutherfurd's drawing of his six-prism spec-
troscope ("American Journal of Science and Arts," 1865),
f See Chapter iv. for an account of the grating.

THE CONSTANTS OF COLOUR. 39

difficult to determine ; probably something might be effected
by carrying out practically the idea suggested in Pig. 6, by
making the necessary additions to the apparatus there in-
dicated. It would be necessary to measure the relative
luminosity of the selected spectral hue and the white light,
and then to mix them in proper proportions, till the mixture
matched the colour of the painted paper, etc. The second
and third colour-constants can be more easily determined.

It may be well here to refer to the terms used to indicate
these constants. For the first constant, the word purity, in
the sense of freedom from white light (or from the sensa-
tion of white), is well adapted. The term luminosity will
be employed in this work to indicate the second constant ;
the third constant will generally be referred to by the term
hue. Colours are often also called intense, or saturated,
when they excel both in purity and luminosity ; for it is
quite evident that, however pure the coloured light may be,
it still will produce very little effect on the eye if its total
quantity be small ; on the other hand, it is plain that its
action on the same organ will not be considerable, if it is
diluted with much white light. Purity and luminosity are,
then, the factors on which the intensity or saturation de-
pends. We shall see hereafter that this is strictly true only
within certain limits, and that an inordinate increase of
luminosity is attended with a loss of intensity of hue or
saturation.

Having defined the three constants of colour, it will be
interesting to inquire into the sensitiveness of the eye in
these directions. This subject has been studied by Aubert,
who made an extensive set of observations with the aid of
coloured disks.* It was found that the addition of one
part of white light to 360 parts of coloui'ed light produced
a change which was perceptible to the eye ; smaller amounts
failed to bring about this result. It was also ascertained

* Aubert, " Physiologie der Netzhaut," Breslau, 1866.

40 MODEBN CHROMATICS.

that- mingling the coloured light of a disk with from 120 to
180 parts of white light (from white paper) caused it to be-
come imperceptible, the hue being no longer distinguishable
from that of the paper.* Differences in luminosity as small
as -j^ to ilir could also, under favourable circumstances, be
perceived. It hence followed that irregularities in the illu-
mination, or distribution of pigment over a surface, which
were smaller than yfj- of the total amount of light reflected,
could no longer be noticed by the eye. Experiments with ,
red, orange, and blue disks were made on the sensitiveness
of the eye to changes of hue or wave-length ; thus, the
combination of the blue disk with a minute portion of the
red disk altered its hue, moving it a little toward violet ;
on reversing the case, or adding a little blue to the red disk,
the hue of the latter moved in the direction of purple.f
Similar combinations were made with the other disks. Au-
bert ascertained in this way that recognizable changes of
hue could be produced, by the addition of quantities of
coloured light, as small as from -j-J^ to ^^ of the total
amount of light involved. From such data he calculated
that in a solar spectrum at least a thousand distinguishable
hues are visible. But we can ^till recognize these hues,
when the light producing them is subjected to considerable
variation in luminosity. Let us limit ourselves to 100
slight variations, which we can produce by gradually in-,
creasing the brightness of our spectrum, tUl it finally is
five times as luminous as it originally was. This will fur-
nish us with a hundred thousand hues, differing perceptibly
from each other. If each of these hues is again varied

* To obtain correct results it is of course necessary to know the lumi-
nosity of the coloured disk aa compared with the white disk, for in the above
results by Aubert they are considered to be equal. With the aid of the
table of luminosities previously given, this correction can be made, and it
will be found that four or five times as much white light is actually neces-
sary as is indicated above.

f Compare Chapter x.

THE CONSTANTS OF COLOUK. 41

twenty times, by the addition of different quantities of
white light, it cames the number of tints we are able to
distinguish up as high as two millions. In this calculation
no account is taken of the whole series of purples, or of
colours which are very luminous or very dark, or mixed
with much white light.

To the above we may add that interesting experiments
on the sensitiveness of the eye to the different spectral
colours have also been made by Charles Pierce, who found
that the photometric susceptibility of the eye was the same
for all colours. (See "American Journal of Science and
Arts," April, 1877.)

With the aid of Vierordt's measurements previously
given, and the determinations by the author of the spaces
occupied by the different colours in the spectrum, a very
interesting point can now be settled, viz. : we can ascertain
in what proportions the different colours are present in
white light. The amount of red light, for example, which
is present will evidently be equal to the space which it oc-
cupies in the spectrum, multiplied by its luminosity, and
the same will be true of all the other colours. The author
constructed a curve representing Vierordt's results, and
from this, taken in combination with his own determina-
tions of the extent of the coloured spaces, obtained the
following table :

Table showing the Amounts or Coloured Light in 1,000 Pakis of White
Sunlight.

Red 54

Orange-red 140

Orange 80

Orange-yellow 114

Yellow 84

Greenish-yellow 206

Yellowish-green 121

Green and blue-green 134

Cyan-blue. 32

43 MODERN CHROMATICS.

Blue ,■•■ 40

Ultramarine and blue-violet 20

Violet 5

1,000

Artists are in the habit of dividiag up colours into warm
and cold. Now, if we draw the dividing line so as to in-
clude among the warm colours red, orange-red, orange,
orange-yeUow, yellow, greenish-yellow, and yellowish-green,
then in white light the total luminosity of the warm colours
will he rather more than three times as great as that of the
cold colours. If we exclude from the list of warm colours
yellowish-green, then they will be only about twice as
luminous as the cold. We shall make use of this table
hereafter.

It may have seemed strange that the chrome-yellow
paper previously mentioned reflected eighty per cent, of
light (the reflecting power of white paper being 100), while
the table just given states that white light contains only a
little more than five per cent, of pure yellow light. It will,
however, be shown in a future chapter that chrome-yellow
really reflects not only the pure yellow rays, but also the
orange-yellow and greenish-yellow, besides much of the red,
orange, and green light. By mixture, all these colours
finally make a yellow, as will be explained in Chapter X.
The high luminosity of some of the other coloured papers
is to be explained in a similar manner.

CHAPTER IV.

PRODUCTION OF COLOUR BY mTEEFERENOE AND
POLARIZATION.

Iw Chapter II. we studied the spectral tints produced by
a prism and by a grating ; these were found to be pure and
brilliant as well as very numerous, and consequently were
adopted as standards for comparison. Most nearly allied
to these central normal colours are those which are pro-
duced by the polarization of light. In this class we meet
with a far greater variety of hues than is presented by the
solar spectrum ; and, instead of a simple arrangement of
delicately shading bands, we encoxmter an immense variety
of chromatic combinations, sometimes worked out with ex-
quisite beauty, but as often arranged in a strange fantastic
manner, that suggests we have entered a new world of
colour, which is ruled over by laws quite different from
those to which we have been accustomed. And it is indeed
so ; the tints and their arrangement depend on the geo-
metrical laws which build up the crystal out of its mole-
cules, and on the retardation which the waves of light ex-
perience in sweeping through them, so that in the colours
of polarization we see, as it were, Nature's mathematical
laws laying aside for a moment their stiff awkwardness, and
gayly manifesting themselves in play.

The apparatus necessary for the study of these fasci-
nating and often audacious colour-combinations is not
necessarily complicated or very expensive. A simple form

MODERN CHROMATICS.

of polariscope is shown in Fig. 11. It consists merely of
a plate of window-glass at P, which is so placed that the
angle, a, is 33° as nearly as possible ; at N is a Nicol's
prism, and at L a plano-convex lens with a focal length of
about an inch. The distance of the prism from the plate

Fig. 11.— Simpio Polarizing Apparatus.

of glass is ten inches ; the lens is removable at .pleasure,
the Nicol's prism is capable of revolving around its longer
axis. If, now, light from a white cloud be reflected from
the plate of glass toward the Nicol's prism, as indicated by
the arrow, some of it will ordinarily traverse the prism and
reach the eye at E ; the prism -should now be turned till
this light is cut off, and the instrument is then ready for
use. Thin slips of selenite or crystals of tartaric acid
placed at 8, so that they are magnified, display the colours
of polarization very beautifully. The arrangement just
described constitutes a simple polarizing microscope ; if a
compound polarizing microscope can be obtained, it will be
still more easy to study the colours and combination of
colours presented by the crystals bf many different salts.
By dissolving a grain or two of the substance in a drop of
watet, and allowing it to crystallize out on a slip of glass, it
is possible very easily to make objects suitable for exami-
nation.

Thin plates of selenite, obtained by removing successive
layers with a penknife, answer admirably if we wish to
study the phenomena of chromatic polarization in their
simplest forms. It will often be found that nearly the

PRODUCTION OF COLOUR BY INTERFERENCE, ETC. 45

whole plate presents a single unshaded hue, which bears a
close resemblance to a patch of colour taken from some por-
tion of the spectrum. But, however bright the colour may
be, it is never free from an admixture of white light, and
it is the constant presence of this foreign element which
causes the colours of polarization to appear a little less in-
tense than those of the spectrum. With plates which are
somewhat thicker the proportion of white light increases,
washing out the colour, till only a faint reminiscence of it
is left. The more powerful tints, however, quite equal and
probably surpass in purity — that is, in freedom from white
light — the most intense sunset hues. Among these colours
we find many shades of red and purple-red ; all the red-
orange hues are represented, and the same is true of the
other colours found in the spectrum. Besides, there is a
range of purples which bridges over the chasm between the
violets and reds ; faint rose-tints are also present in abun-
dance, and the same is true of the pale greens and bluish-
greens. In addition to this, quite thin slips furnish a dis-
tinct set of tints which are peculiar in appearance, and
which, when once seen, are never forgotten ; a singular
tawny yellow will be noticed in combination with a bluish-
grey ; the yellow, such as it is, shading into an orange
nearly allied to it, and this again into a brick-red ; black
ai.d white will be associated with these subdued tints ; the
general impression produced by these combinations being
sombre if not dreary.

These slips of selenite furnish neither beautiful nor com-
plicated patterns, the tints being for the most part arranged
in parallel bands, with here and there angular patches, often
in sharp contrast with the other masses of colour. There is
no noticeable attempt at chromatic composition, except per-
haps a little along fractured edges, where we frequently
meet with pale grey or white deepening, into a fox-coloured
yellow, followed by a red-violet, brightening into a sea-
green dashed with pure ultramarine; or changing Suddenly

46 MODERN CHROMATICS.

into a full orange-yellow, after which may follow a broad
field of purple. Just as often all the tints are pale, like
those used on maps, with a narrow fringe on the edge, of
rich variegated hues. The colour-combinations seldom rise
into great beauty, though they often astonish and dazzle
by their audacity and total disregard of all known laws of
chromatic composition. The brilliancy and purity of these
tints are so great, and they are laid on with such an unfal-
tering hand, that all these wild freaks are performed com-
paratively with impunity, and it is only when we proceed
to make copies of these strange designs that we become
fully aware of their peculiarities, and, from an artistic point
of view, positive defects.

Crystals of tartaric acid present phenomena which are
quite different : here the patterns are rich and often beautiful;
the colour is full of gradation, touched on and retouched
and wrought out with patience in delicate, complicated
forms, which echo or faintly oppose the grand ruling ideas
of the composition. We may have a wonderfully shaped
mass radiating in curved lines over the entu-e field, tinted
with soft grey and pale yellow, with here and there dashes
of colour like the spots on a peacock's tail, glowing like
coals of fire ; all this being set off by very dark shades of
olive-green, dark browns and greys. If the crystals are thin
this is their appearance ; but as the thickness increases so
does the brilliancy of the hues, which are sure to be well
contrasted with large masses of deep shade. The soft gra-
dations, the sharp contrasts, the brilliant and pale colours,
the dark shadows and the wonderful forms, all combine to
lend to these pictures a peculiar charm which is not wholly
lost even in copies executed in ordinary pigments.

Common sugar, if allowed time to crystallize out slowly,
furnishes appearances somewhat resembling the above, but
the designs are more formal and less interesting. Crystals
of nitrate of potash present appearances, again, which are
totally unlike those above mentioned ; here we have a great

PRODUCTION OF COLOUR BY INTERFERENCE, ETC. 47

number of delicately tinted threads of light ; there will be
purples and golden greens or dull olive-greens and carmines,
woven together so closely as almost to produce a neutral
tint, which will brighten suddenly and display combinations
of pui-ple-red with green, dashed here and there with pure
ultramarine. These tinted threads of light will be disposed
with regularity as though it had been intended to weave
them into some wonderful cashmere-like pattern, and then
warp and woof had been suddenly abandoned and for-
gotten.

It would be useless to multiply these descriptions — every
salt has its own peculiarities and suggests its own train
of fancies ; some glow like coloured gems with polished
facets, or bristle with golden spears like the advancing ranks
of two hosts in conflict, or suggest a rich vegetation made
of gold and jewels and bathed in sunset hues. Artists who
see these exhibitions for the first time are generally very
much impressed by their strange beauty, and not unfre-
quently insist that their range of colour-conceptions has
been enlarged. It has often seemed to the author that the
cautious occasional study of some of these combinations
might be useful to the decorator in suggesting new concep-
tions of the possibilities within his reach.

When polarized light is made to traverse crystals in the
direction of -their optic axes, phenomena of a different kind
are presented. They were discovered in 1813 by Brewster,
and, on account of their scientific interest and a certain
beauty, have since then greatly attracted the attention of
physicists and even of mathematicians. A series of rainbow-
like hues, disposed in concentric circles, is seen on a white
field ; a dark-grey cross is drawn across the gayly coloured
circles, and, after dividing them in four quadrants, fades
out in the surrounding white field. By a slight change in
the adjustment of the apparatus, the grey cross can be made
white ; the rings then assume the complementary tints.
Other crystals, again, furnish double sets of rings, the daris

48 MODERN CHROMATICS.

cross being shared by tliem jointly, or so altered in form as
no longer to be recognizable.

These appearances have been considered by many phys-
icists to be extraordinarily beautiful ; it is, however, to be
suspected that in this case the judgment was swayed by
other considerations than those of mere beauty. The rarity
of the phenomenon, the difficulty of exhibiting it; the bril-
liant list of names identified with it, along with the insight
it furnishes as to the molecular constitution of crystals, all
combine to warp the judgment, and to seriously influence
its final award. In point of fact, the formal nature of the
figures, the constant repetition of the rainbow-tints in. the
same set order, which is that of the spectrum, both exclude
the possibility of the charming colour-combinations so fre-
quently presented by many salts when simply crystallized
on a 'slip of glass. The cross and rings are not for a mo-
ment, in matter of beauty, to be compared with the appear-
ances presented by crystals of tartaric acid.

Glass which has been heated and then suddenly cooled,
or glass which is under strain, exhibits phenomena of coloxir
closely related to the above ; we have as it were a set of
distorted crosses and rings which sometimes lend themselves
more kindly to the production of chromatic effects than is
the case with the normal figures.

In ordinary life the colours of polarization are never
seen ; the fairy world where they reign cannot be entered
without other aid than the unassisted eye. This is not a
matter for regret ; the purity of the hues and the audacious
character of their combinations cause their gayety to appear
strange and unnatural to eyes accustomed to the far more
sombre hues appropriate to a world in which labour and
trouble are such important and ever-present elements. The
colours even of flowers have a thoughtful cast, when com-
pared with those of polarization.

The colours which have just been considered are pro-
duced in a peculiar manner ; the complete explanation is

PRODUCTION OF COLOUR BY INTERFERENCE, ETC. 49

long and tedious, and has for us no particular interest. The
main idea, however, is this : white light is acted on in such
a way that one of its constituents is suppressed ; the result
is coloured light. For example, if we strike out from white
light the yellow rays, what remains will produce on us the
sensation of blue ; if we cut off the green rays, the remain-
der will appear purple. The reason of this will be more
fully appreciated after a study of the facts presented in
Chapters XI. and XII. To effect this sifting out of certain
rays a polarizing apparatus is employed ; when the crystals
are removed from it, the colour instantly vanishes. Now,
it so happens that there is a class of natural objects capable
of displaying exactly the same hues without the intervention
of any piece of apparatus — all objects that fulfill a certain
condition may be reckoned in this class ; it is merely that
their thickness should be very small. Thin layers of water,
air, glass, of metallic oxides, of organic substances, in fact
of almost everything, display these colours. The most fa-
miliar example is furnished by a soap-bubble. When it first
begins to grow, it is destitute of colour and perfectly trans-
parent ; it gives by reflection from its spherical surface a
distorted image of the window, with the bars all curved,
but no unusual hues are noticed till it has become somewhat
enlarged.* Then faint greens and rose-tints begin to make
their appearance, mingling uneasily together as if subjected
to a constant stirring process. As the bubble expands and
the film becomes more attenuated, the colours gain in bril-
liancy, and a set of magnificent blue and orange hues, pur-
ples, yellows and superb greens replaces the pale colours
which marked the early stages, and by their changing flow
and perpetual play fascinate the beholder. If the bubble

* It is not very uncommon to meet with paintings in wliicli a bubble
has been represented with window-bars on its surface, where nothing of
the kind could have been visible. A friend has mentioned to the author
four cases where dififerent ai-tists have introduced window-bars instead of
slty and landscape, on the surfaces of bubbles which were in the open air.

50 MODERN CHROMATICS.

has the rare fortune to live to a good old age, at its upper
portion a different series of tints begins to be developed ;
the tawny yellow, before mentioned, begins to be seen in
irregular patches, floating around among the more brilliant
hues, a sign that the attenuation has nearly reached its ex-
treme limit ; but, if by some unusual chance it should be a
Methuselah among bubbles, pale white and grey tints also
are seen, after which it is sure to burst. A long-lived soap-
bubble displays every colour which can be produced by
polarization. The thin film has a sifting action on white
light, which in its final result is the same as in the case of
the production of colour by polarized light : certain rays are
struck out, and, as before, white light deprived of one of
its constituents furnishes coloured light. This elimination
is accomplished by the interference of the waves of light in-
volved ; hence, colours produced in this way are called " in-
terference colours." The colours of polarization are also just
as truly interference colours, but they are not usually known
under this name. From all this it follows that the colours
produced by thin layers, or by very fine particles, always
contain some white light, and consequently cannot quite
rival in purity or intensity the spectral hues.

The colours of polarization, as we have seen, are never
met with outside of the laboratory. Nature, on the other
hand, here and there with a sparing hand, displays in small
quantity, and as a rarity, the colours of interference. They
are used as a wonderful kind of jewelry in the adornment
of many birds ; lavishly so in the case of the common pea-
cock, where the breast and tail feathers in full sunshine dis-
play flashing, dazzling hues, which make our artificial or-
naments appear pale and tame. In. contemplating these
astonishing hues, or those of that tiny winged jewel, the
humming-bird, we are struck by the circumstance that they
actually have a metallic brilliancy, which we in vain at-
tempt to rival with our brightest pigments. To compete
with them successfully,, it is necessary to substitute a sur-

PRODUCTION OF COLOUR BY INTERFERENCE, ETC. 51

face of silver for the white paper, and to cover it with the
purest and most transparent glazes. This appearance of
metallic lustre depends on the circumstance that much col-
oured light is reflected, mingled with only a small quantity
of white light, the great bulk of the latter being absorbed
by the dark pigment contained in the interior of the feath-
ers. When this dark pigment is absent, we have as before
colour ; but, being mingled with much reflected white lightj
it- presents simply an appearance like that of mother-of-
pearl.

There is yet another peculiarity of the colours now
under consideration, which still more completely separates
them from the hues furnished by pigments : it is their va-
riability. These colours, as has been mentioned, are pro-
duced by the interference of the waves of light which are
reflected from the thin films : the nature of this interference
depends partly on the angle at which this reflection takes
place, so that, as we turn a peacock's feather in the hand,
its colour constantly changes. The same is true of the tints
of the soap-bubble, and of interference colours in general
— the hue changes with the position of the eye ; as they are
viewed more and more obliquely, the tint changes in the
order of the spectrum, viz., from red to orange, to yellow,
etc.

The brilliant metallic colours exhibited by many insects,
particularly the beetles, belong also in this class, so also the
more subdued steel-blues and bottle-greens displayed by
many species of flies. So commonly does this occur that it
suggests the idea that these humble creatures are not desti-
tute of a sense for colour capable of gratification by bril-
liant hues. If we descend into the watery regions we find
their inhabitants richly decorated with colours of the same
general origin, the pearly rainbow hues which they display
all depending on the interference of light. The same is
true of the iridescent hues which so commonly adorn shells
externally and internally. In this case candour compels one

52 MODERN CHROMATICS.

to- admit that the colours, beautiful as they are, can hardly
be a source of pleasure to the occupants or to their friends.

Leaving the animated world, we find the colours of in-
terference shown frequently, but in an inconspicuous man-
ner, by rather old window-glass ; some of the alkali seems
to be removed by the rain, and in the course of time a thin
film of silica capable of generating these hues is formed.
In antique glass which has long remained bm-ied this pro-
cess is carried much further, so that sometimes the whole
plate or vase tends to split up into flates. Here, owing to
successive reflections on many layers, the light which reach-
es the eye is quite bright, and the colours intense. Crim-
son, azure, and gold are found in combination ; blue melts
into purple or flashes into red ; ruby tints contrast with
emerald hues : each change of the position of the eye or of
the direction of the light gives rise to a new and startling
effect. In other cases broad fields of colour, with much
gentle gradation and mingling of tender pearly hues, re-
place the gorgeous prismatic tints, and fascinate the be-
holder with their soft brilliancy.

The iridescent hues of many minerals fall into the same
general class ; they are beautifully displayed by some of
the feldspars, and the brilliant hues found on anthracite
coal have also the same origin. The blue films often pur-
posely produced on steel are due to thin layers of oxide of
iron which suppress the yellow rays. Other cases might be
mentioned, but these will sufiice for the present.

CHAPTER V.
ON THE COLOURS OF OPALESCENT MEDIA.

If wHte light be allowed to fall on water which is con-
tained in a clear, colourless glass vessel, some of it will be
reflected from the surface of the liquid, while another por-
tion will traverse the water and finally again reach the au-.
These well-known facts are represented in Fig. 12. An
eye placed at E will perceive the reflected light to be white,
and the transmitted light will also appear white to an eye
situated at O. But, if now a little milk be added to the
water, a remarkable change will be produced : light will, as
before, be reflected from the surface to the eye placed at E,
and this surface-light will still be white ; but the little milk-
globules ' under the surface and throughout the liquid will
also reflect light to E — this light will be bluish. From this
experiment, then, it appears that the minute globules sus-
pended in the liquid have the power of reflecting light of a
bluish tint. In Fig. 12 the light is represented as being
reflected only in one direction ; but, when the milk-globules
are added, they scatter reflected light in many directions, so
that an eye placed anywhere above the liquid perceives this
bluish appearance.

On the other hand, after the addition of the milk, the
light at O (Fig. 12), which has passed through the milky
liquid, will be found to have acquired a yellowish tint.
From this it appears that fine particles suspended in a liquid
have the power of dividing white light into two portions,
tinted respectively yellowish and bluish. If more milk be

MODERN CHROMATICS.

added to the water, white light will mingle in and wiU final-
ly overpower the bluish reflected light, so that it will hardly
he noticed ; as the quantity of mUk is increased, the colour
of the transmitted light will pass from yellow to orange, to
red, and finally disappear, the liquid having become at last
so opaque as to cease to transmit light altogether.

Fi8. 12.— Kefleotion and Transmission of Light by Water.

This very curious action is not confined to mixtures of
milk and water, but is exhibited whenever very fine parti'
cles are suspended in a medium different from themselves.
If an alcoholic solution of a resin is poured with constant
stirring into water, very fine particles of resin are left sus-
pended in the liquid, and give rise to the appearances just
described. BrUcke dissolves one part of mastic in eighty-
seven parts, of alcohol, and then mixes with water, the water

ON THE COLOURS OF OPALESCENT MEDIA. 55

being kept in constant agitation. A liquid prepared in tHs
way shows by reflected light a soft sky-like hue, the colour
of the light which has passed through being either yellow
or red, according to the thickness of the layer traversed.
The suspended particles of resin are very fine, and remain
mingled with the water for months ; they are often so fine
as to escape detection by the most powerful microscopes.

Some kinds of glass which are used for ornamental pur-
poses possess the same property, appearing bluish- white by
reflected light, but tingeing the light which comes through
them red or orange-red. The beautiful tints of the opal
probably have the same origin, and the same is (rue also of
the bluish, milky colour which characterizes many other
varieties of quartz.

Not only liquids and solids exhibit this phenomenon of
opalescence, but we find it also sometimes displayed else-
where ; thus, for example, a thin column of smoke from
burning wood reflects quite a proportion of blue light,
while the sunlight which traverses it is tinted of a brown-
ish-yellow, or it may be, even red, if the smoke is pretty
dense.

AH these phenomena are probably due to an interference
of light, which is brought about by the presence of the fine
vparticles, the shorter waves being reflected more copiously
than those which are longer ; these last, on the other hand,
being more abundantly transmitted. An elaborate expla-
nation of the mode in which the interference takes place
would be foreign to the purpose of the present work ; we
therefore pass on to the consideration of the more practical
aspects of this matter.*

It will be well to notice, in the first place, certain con-
ditions which favour not so much the formation as the per-
ception of the tints in question : thus it will be found that

* Compare E. Briioke, in Poggendorff 's " Annalen," Bd. 88, S. 363";
alBO Bezold's " Farbenlehre," p. 89.

MODERN CHROMATICS.

the blue tint, in the experiments with the liquids, is best
exhibited by placing the containing glass vessel on a black
surface. This effectually prevents the-blue reflected light
from being mingled with rays which have been directly
transmitted from underneath. Indeed, the mere presence
or absence of a dark background may cause the tint which

Fig, 13. — Smoke appears Blue on a Bark Backgroimd ; Brown on a Light Background.

finally is perceived by the eye to change from yellow to
blue, the other conditions all remaining unaltered. Thus in
Fig. 13 we have thin smoke seen partly against a dark
background, and partly against a sky covered with white
clouds : the lower portion is blue, from reflected light,
while in the upper portion this tint is overpowered by the
greater intensity of the transmitted light, which is yellow-
ish-brown. As a general thing, the reflected and transmit-
ted beams are both present ; dark backgrounds favour the
former, luminous ones the latter.

If a thin coating of white paint, such as white lead or
zinc-white, is spread over a black or dark ground, the
touches so laid on will have a decidedly bluish tint, owing
to the causes which have just been considered. If a draw-
ing on dark paper be retouched with zinc-white used as a
water-colour, the touches will appear bluish and inharmo-
nious, unless especial care is taken to prevent the white

ON THE COLOURS OF OPALESCENT MEDIA. 57

pigment from being to some extent translucent ; this disa-
greeable appearance can only be prevented by making each
touch dense, and quite opaque. For the production of such
effects it is not even necessary to go through the formality
of laying the white pigment on a dark ground ; white lead
mingled with any of the ordinary black pigments gives not
a pure but a bluish grey. This is very marked in the case
of black prepared from cork, which hence has sometimes
been called "beggars' ultramarine." If yellow pigments
are mixed with black, the effect is not simply to darken the
yellow, as would be expected, but it is converted into an
olive-green. This is particularly the case with those pig-
ments which approximate to pure yellow in hue, such as
gamboge and aureolin ; the least admixture of dark pig-
ment carries them over toward green. But if these black-
and-white or black-and-yellow pigments are combined by
the method of rotating disks (see Chapter X.), we' obtain
pure greys or darker yellow tints, showing that the blue
hue is not, as many suppose, inherent in the black pigment,
but an accident due to the mode of its employment. The
above-mentioned cases are marked examples of the applica-
tion of these principles to painting ; but in a more subtile
way the whole theory of the process of oil-painting takes
cognizance of them, and is so adjusted as to avoid difficul-
ties thus introduced, or, more rarely, to utilize them. It is
perhaps scarcely necessary to add that the somewhat bluish
tone of drawings made with body colour, or of frescoes, is
due to these same causes.

Having hinted at the influence which this peculiar op-
tical action exerts on the infancy of a picture, we pass on
to consider some of its effects on a painting in its old age.
It is well known that old oil-paintings frequently become
more or less covered by what seems to be a coating of grey
or bluish-grey mould, which, spreading itself particularly
over the darker portions, obscures them, so that all details
aje lost, and the work of the artist entirely destroyed. In-

58 MODERN CHROMATICS.

vestigation has shown that this trouble is caused by an iiu'
mense number of fine cracks in the painting itself, which
seem to act somewhat in the same way as the mixtures
which have just been considered, so that the observer is
practically looking at the picture through a rather dense
haze. By filling these invisible cracks up with varnish the
matter is somewhat helped, but much moi'e perfectly by. the
"regenerations process" of Pettenkofer. This celebrated
chemist once by accident used an old, worn-out oil-cloth
mat, from which the pattern had mostly disappeared, to
cover a beaker containing hot alcohol. On removing the
mat, some hours afterward, he was surprised to find that
the portion acted on by the alcoholic vapours had been reju:
venated and the pattern restored. It was soon ascertained
that the vapours had softened the pigment, and the separated
grains had again been fused together. Experiments on old
oil-paintings gave similar results, and the process is now in
use in some of the largest European galleries.

In many other objects besides those that have been
mentioned these peculiar tints can be observed ; among
minor examples may be mentioned the bluish-grey or green-
ish-grey tint which marks the course of veins under the
skin ; the blue or greenish colour of the human eye also
owes its tint to the same cause. In these cases an opalescent
membrane is spread over a dark background, and the colour
is produced in the same manner as in the experiments de-
scribed at the commencement of this chapter. In blue eyes
there is no real blue colouring-matter at all.

It is, however, the sky that exhibits this class of tints
on the grandest scale, as well as in the greatest perfection.
Our atmosphere, even when perfectly clear, contains sus-
pended in it immense numbers of very fine particles which
never settle to the earth, and which the rain has no power
to wash down. When they are illuminated by sunlight
they reflect white light mixed with a certain proportion of
blue, and this blue is seen on a black background, which is

ON THE COLOURS OF OPALESCENT MEDIA. 59

nothing less than the empty space in which the earth is
hung. Hence, the blue colour of the sky. This tint on
clear days can be traced up tolerably near the sun, indeed,
until the brightness of the sky. begins to be blinding. An
examination of the deepest blue portions of the sky with
the spectroscope reveals the presence of much white light,
so that the blue colour is very far from being pure or satu-
rated— a fact that young landscape-painters soon have
forced on their attention. In clear weather, as long as the
sun is at a considerable distance from the horizon, the yel-
low colour which accompanies the transmission of light
through an opalescent medium is not much noticed ; but, as
the sun sinks lower, its rays traverse an always increasing
thickness of the atmosphere, and encounter a greater num.
ber of fine particles, so that the transmitted light, late in
the afternoon, becomes decidedly yellow or rather orange-
yellow.

Having thus briefly considered the production of or-
dinary sky-tints, let us pass to the aspects assumed by a
landscape under the influence of the minute suspended par-
ticles. These atoms will of course reflect light toward the
observer, and this light will add itself to that which comes
regularly from objects in the landscape, producing thus
important changes in their appearance. The very thick
layer of air intervening between the observer and the most
distant mountains will send to the eye a very large amount
of whitish-blue light, which will not greatly differ from a
sky-tint. This will entirely overpower the somewhat feeble
light reflected from portions of the mountain in shade, so
that as a result we shall have all the shadows of the moun-
tain represented by more or less pure sky-tints, and these
tints will be far more luminous, far brighter, than the ori-
ginal shadows were. No details will be visible in these
wonderfully shaped bluish patches. Those portions of the
mountains, on the other hand, which are in full sunlight will
still visibly send light to the eye through the haze, and

60 MODEEN CHROMATICS.

their prevailing tint will be yellowish or orange, or some
other -warm tone. Not many details will be visible, and the
actual colours or local colours of the mountain will not at
all. ai^pear, or at most will only blend themselves with the
soft, warm tints due to the medium. In a word, the con-
trast between the light and shade will be enormously dimin-
ished, so that the general luminosity of the mpuntain. will
be hardly less than that of the sky itself, and its colour will
be worked out mainly in tints which have the same origin
and character with those of the sky. As we approach
nearer the. mountain, these effects begin slowly to- diminish,
and in the sunlit portions delicate greens, varied and soft
greys, begin to make their appearance, while the shadows
lose theh' heavenly blue, and, darkening, become bluish-
grey. Afterward all those parts lying in sunlight display
their local tints, somewhat softened, and the coloured light
from the shadows begins to make itself felt, and, mingling
with the blue reflected light, to produce soft purplish-greys,
greenish-greys, and other nameless tints. The sunlit por-
tions of the pine-trees will be of an olive-green or of a low
greenish-yellow, the shadows on the same trees being pure
grey or bluish-grey without any suggestion of green. On
nearer objects these effects are less traceable, and the natural
relation between light and shade more and more preserved,
so that contrast of this last kind becomes progressively
stronger as we turn from the most distant to the nearest
objects. All these effects are readily traceable on any or-
dinary clear day ; they change, of course, with the condition
of the atmosphere, and as it becomes misty the blue reflected
light changes to grey, the transmitted light not being
equally affected.

Late in the afternoon, when the sun is low, its rays trav-
erse very thick layers of the atmosphere, and wonderful
chromatic effects are produced. Near the sun the transmit-
ted light is yellowish, but so bright that the colour is not
very perceptible ; to the right and left the colour deepens

ON THE COLOURS OF OPALESCENT MEDIA. 61

into an orange, often into a red, whicli, as the distance from
the sun increases, fades out into a purplish-grey, greyish-
blue, passing finally into a sky-blue. The warm tints are
produced mainly by transmitted light, the cold ones by re-
flected light, and the neutral hues by a combination of both.
Above the sun there is, in clear sunsets, a rather regular
transition upward, from the colours due to transmitted, to
those produced by reflected light. As the sun sinks lower
its rays encounter a greater mass of suspended particles,
and the warm tints above mentioned move toward the red
end of the spectrum, and also gain in intensity. The pres-
ence of clouds breaks up the symmetry of these natural
chromatic compositions, and gives rise to the most magnifi-
cent effects of colour with which we are acquainted. The
landscape itself sympathizes with the sky, and near the sun,
chameleon-like, assumes an orange or even red hue ; while at
greater distances its cold tints are wanned, even the greens
being changed into olive or yellowish hues. Simultaneous-
ly the shadows lengthen enormously, bringing thus the com-
position into grand and imposing masses, and investing even
the most commonplace scenery with an air of great noble-
ness and beauty.

The complete series of sunset hues, from the brightest
light to the deepest shade, runs as follows :

1-. Yellow. I 3. Red. I S. Violet-blue.

2. Orange. I 4. Purple. ' 6. Grey-blue.

This, as it were, normal series is often interrupted by
the omission of one or more of the intermediate hues, and
sometimes begins as low as the red or even purple.

CHAPTER YI.

PRODUCTION OF COLOUR BY FLUORESCENCE AND
PHOSPHORESCENCE.

In all the cases thus far examined, colour has heen pro-
duced either by the analysis of white light or by subjecting
it to a process of, subtraction, as in the examples mentioned
in the last chapter. The very astonishing discovery of
Stokes, however, has proved that colour can be produced in
a new and entirely different way. If, in a darkened room
the pure violet light of the spectrum be allowed to fall on
a plate or wineglass made of uranium glass, these articles
will not reflect violet light to the eye as would be expected,
but will glow with a bright-green light, looking in the dark-
ness almost as though they had suddenly become self-lumi-
nous. This kind of glass has, then, the extraordinary prop-
erty of entirely altering the colour of the light that falls on
it, and of causing the light to assume a quite different
hue. But, as colour depends on wave-length, we are led to
ask whether this property of the original beam of light is
also affected by the uranium glass. Stokes proved conclu-
sively that this is the case, and that in all such experiments
the length of the wave is made greater. It would appear
that the waves of light act on the atoms which make up (or
surround) the molecules of the glass, and set them in vibra-
tion ; they continue in vibration for some little time after-
ward, at a rate of their own selection, which is always less
than that of the waves of light which gave the first impulse.
Being in vibration, they act as luminous centres, and com-

PRODDCTION OF COLOUR BY FLUORESCENCE, ETC. 63

municate vibrations to the external ether, and this is the
green light that finally reaches the eye. The action takes
place not only on the surface of the glass, but deep in its
interior, so that, if the experiment be made with a thick
cube of the glass, it actually appears milky and almost
opaque, owing to the abundant flood of soft green light
which it pours out in all directions. It is not even neces-
sary to employ as a source of illumination the pure violet of
the spectrum ; sunlight streaming through blue cobalt glass
answers as well, and the sharp change from the violet-blue
to the milky-green is quite as astonishing.

Under ordinary daylight uranium glass scatters in all
directions a bluish -green light, which is due to the cause
above mentioned, but the light which passes through its
substance is merely tinged yellow. Both these tints make
their appearance in daylight, and by their combination
communicate to articles made of this glass a peculiar and
rather beautiful appiearance. Candle-light or gas-light fur-
nishes but a scanty supply of blue and violet rays, hence
this kind of illumination robs uranium glass entirely of
its charm, and the articles made of it assume a dull yel-
lowish hue which is neither striking nor attractive. There
are many salts which have this property in a high degree :
among the best known is the platino-cyanide of barium,
which presents appearances similar to those above men-
tioned. Thallene, an organic substance derived from coal-
tar, and described by Morton, must also be classed with
uranium glass.* Drawings made with this substance on
white paper, by daylight appear yellowish, but when placed
under a violet or blue illumination flash into sudden bril-
liancy, and scatter in all directions a strong greenish light.
There are many liquids which have the same property,
and which display different colours when acted on by violet
light, but for an account of them we must refer the curi-

* See "Chemical News," December, 1S72.

64 MODERN' CHKOMATICS.

ous reader to. Dr. Pisko's work on the fluorescence of
light.*

Before passing from this subject it maybe as well to. add
.that phosphorescence also often gives rise to colours which
more or less closely resemble those of fluorescence. If tubes
filled with the sulphides of barium, strontium, calcium, etc.,
be placed in a dark room and illuminated for an instant
by a beam of sunlight, by the electric light, or by bumiag
magnesium wire, they will display a charming set of tints
for some minutes afterward. Some will shine with a soft
violet light, others will display an orange or yellow glow ;
delicate blues will make their appearance, and will contrast
well with the red hues, the latter resembling in the dark-
ness living coals of fire. The tints, as such, are very beau-
tiful and suggestive, though of course no direct application
can be made of them to artistic purposes.

* " Die Fluorescenz des Lichtes," F. J. Pisko, Yicnna.

CHAPTER VII.

ON THE PRODUCTION OF COLOUR BY ABSORPTION.

The colours produced by the dispersion, interference,
and polarization of light have great interest from a purely
scientific point of view, and are also valuable in helping us
to frame a true theory of colour, but it is to the phenomena
of absorption that the colours of ordinary objects are almost
entirely due. The pigments used by painters, the dyes em-
ployed by manufacturers, the colouring-matter of flowers,
trees, rocks, and water, all belong here. Let us begin our
study of this subject with a fragment of stained glass.
When we place the glass flat on a surface of black cloth,
and expose it to ordinary daylight, we find that it reflects
light to the eye just as a piece of ordinary window-glass
would under similar circumstances, and this light is white,
not coloured. In this experiment, the rays of light which
reach the eye have been reflected from the mere surface of
the plate of glass, those rays which penetrate its interior
being finally absorbed by the black cloth underneath, and
never reaching the eye at all. If we now raise the glass
and allow the light of the sky to pass through it and fall
on the eye, we find that it has been coloured ruby-red. The
light of a candle- or gas-flame is affected in the same way,
and a beam of sunlight streaming through the plate of glass
falls on the opposite wall as an intensely red, luminous
spot. Our first and very natural impression is, that the
stained glass has the power of altering the quality of light —
that the white light is in some way actually transmuted

MODERN CHROMATICS.

rCito red light. This seems to he the universal impression
among those who have not particularly examined the mat-
ter. We saw, in Chapter II., that with a prism we could
analyze white light, and sort out the waves composing it
according to their length, and that the sensation which- the
waves produced on the eye varied with their length, the long-
est giving red, the shortest violet. The prism can also be ap-
plied to the study of the matter now under consideration. A

Fig. 14.— Eed Glass placed over Slit in Black CardbQard,

screen of black pasteboard is to be prepared with a narrow
slit cut in its centre ; over the slit a piece of stained glass is
to be fastened, as indicated in Fig. 14. If, now, this arrange-
ment be placed in front of a window, matters can be so
contrived that white light from a cloud shall fall upon the
slit and traverse the stained glass ; it will afterward reach

Pio. IB.— Spectrum of Light transmitted through Eed Gllass.

the prism which will analyze it. On mating this experi-
ment we find that the result is similar to what is indicated
in Fig. 15 : the prism informs us that the transmitted beam

ox THE PRODUCTION OP COLOUR BY ABSORPTION. 67

consists mainly of red light ; a little orange light is also
present. The experiment can, howe%''er, be made in a more
instructive way, by covering only half of the slit with
the plate of glass. On repeating it with this modification,
we obtain side by side an analysis of the white light direct
from the cloud, and of the light which has traversed the
ruby glass ; the result is indicated .in Fig. 16, and we see

■ RIO

/EL (

E
REEN

BLUE

VIOLET 1

1 RED

mm^

^^1

im^H

^^HHHi

Fia. 16.— Spectrum of White and Ked Light compared.

at a glance that the solution of the whole matter is sim-
ply this : the ruby glass is able to transmit the red rays,
but it stops all the others ; these last it absorbs — hence
we say it produces its colour by absorption. The other rays
are in fact converted into heat, and raise the temperature
of the glass to a trifling extent. The experiment can be
varied somewhat without affecting the result ; if a solar
spectrum be projected on a screen, as described in Chapter
II., we shall find, when we look at it through the ruby glass,
that we can see only the red space, light from the other col-
oured spaces not being able to pen'etrate the glass ; and
finally, when we hold our plate of glass directly in the paths
of the coloured rays, we shall notice that it stops all ex-
cept those that are red. These simple fundamental experi-
ments prove that the ruby glass does not transmiite white
light into red, but that it arrests certain rays, and converts
them into a kind of force which has no effect on the eye ;
the rays which are not arrested finally reach the eye and
produce the sensation of colour.

MODERN CHROMATICS.

For more careful examinations of the coloured light
transmitted by stained glass a spectroscope with one flint-
glass prism can advantageously be used. The stained glass
is to be fastened so that it covers one half of the slit, and
then we shall have, placed side by side, the spectrum due
to the glass and a prismatic one for comparison. In this
latter the fixed lines will be present, and we can use them as
a kind of natural micrometer for mapping down our results.
There is, however, another point to be attended to. When
we come to examine the red glass carefully with the spec-
troscope, we find that it not only transmits the red rays
powerfully, but that a little of the orange rays also passes
through with still smaller portions of the gi-een and blue
rays. Hence we are dealing not only with spaces in the
spectrum, but with the relative intensities of the coloured
light filling those spaces. It is difficult, or rather impos-

RED

YCL. GBEEM

BLUE

yiQlET

Fio. 17.— Spectrum, showing the Extent and Intensity of the Golonred Light transmitted
by Eed Glass. The shaded portion represents the transmitted hght.

sible, to represent the different intensities by shading on
paper ; hence physicists have adopted a certain convention
which removes this trouble, and enables them to express
differences in luminosity readily and accurately. All this
is accomplished by drawing a curve, and agreeing that dis-
tances measured upward to it shall represent different de-
grees of luminosity. We agree, then, to let the entire rec-
tangle A H O N, Fig. 17, represent a solar spectrum, with

ON THE PRODUCTION OF COLOUR BY ABSORPTION. 69

its different colours properly arranged, and having their
natural or normal luminosities, and in this rectangle we
draw the curve furnished by the red glass (Fig. 17). We
find that it is highest in the red space ; but even here it
reaches only about half way up, showing that the luminos-
ity of the transmitted red light is only half as great as that
of the same light in the spectrum ; in the orange space it
falls rapidly off, the curve sinking with a steep slope ; after
that it runs out into the green and blue, almost to the vio-
let, in such a way as to indicate that the red glass transmits
minute quantities of these different kinds of coloured light.
The luminosity, then, of all the transmitted rays, except
the red, being quite feeble, the light which comes through
appears pure red. Making an examination of an orange-
yellow glass in the same way, we obtain the curve shown in
Fig. 18 : this glass, it appears, transmits most of the red.

RED

YEL.

GREEN

BLUE

VIOLET

Fio. 18.— The shaded portion shows the amount of light transmitted by an orange-
coloored glass.

orange, and yellow rays, with much of the green and a
little of the blue. Here, of course, the orange and yellow
rays after transmission make up an orange-yellow hue, and
the green and red rays by their union reproduce the same
colour, as we shall see in Chapter X. Hence the final colour
is orange-yellow, without the least tint of red or green.
Taking next a green glass, we obtain another curve. Fig. 19,
showing that much green light is transmitted, but along

MODERN CHROMATICS.

nEO

YEL.

GREEN

BLUE

VIOLET

B C

^ 0

Fig. 19. — The shaded portion represents the amount of light transmitted hy green glass.

with it .some red and some' blue. Blue glass shows the
cyan-blue weakened, the ultramarine-blue and violet strong;
the green is very weak, so also are yellow and orange ; the
red is mostly absent, except a feeble extreme red. The re-
sult is of course a violet-blue (Fig. 20). A purple glass is

RED

YEL.

GREEN

BLUB

VIOLET

Fio. 20.— The shaded portion represents the amount of light transmitted by blue glass.

found to absorb the middle of the spectrum, i. e., the yel-
low, green, and cyan-blue ; the red and violet are also en-
feebled, but are at all events far stronger than the other
transmitted rays. We have, then, as a final result, red,
ultramarine-blue, and violet, which being mingled make
"purple. It is evident from these experiments that the col-
ours produced by absorption are not simple, like those fur-
nished by the prism, but are resultant hues, produced by
the mixture of many different kinds of coloured light hav-

ON THE PRODUCTION OF COLOUR BY ABSORPTION. 71

ing varying degrees of brightness. On this account, and by
reason of the tendency of many kinds of stained glass to
absorb to a considerable extent all kinds of coloured light .
presented to them, it happens that stained glass furnishes
us with coloured light inferior in purity and luminosity to
that obtained by the use of a prism. Nevertheless these
colours are the purest and most intense that we meet with in
daily life, and far surpass in brilliancy and saturation those
produced by dyestuflfs or pigments.

There is one property which probably all substances
possess which produce colour by absorption, upon which a
few words must be now bestowed. If we cause white
light to pass through a single plate of yellow glass, the
rays which reach the eye will of course be coloured yellow.
Add now a second plate of the same glass, and the light
which traverses the double plate assumes a somewhat dif-
ferent appearance ; it evidently is not so luminous, and its
colour is no longer quite the same. Using six or eight
plates of the yellow glass, we find that the transmitted
light appears orange. If the same experiment be repeated,
using a considerable number of plates of the same glass,
the colour will change to dark red. From this it appears
that the colour of the transmitted beam of light depends
somewhat on the thickness of the absorbing medium. This
change in the case of some liquids is very considerable :
thin layers, for example, of a solution of chloride of chro-
mium transmit green light mainly, and so imitate the ac-
tion of a plate of green glass ; thick layers of the same
liquid transmit less light in general, but the dominant
colour is red, and objects viewed through them look as
they would, seen through a plate of dark-red glass. This
curious property is easily explained by an examination of
the action of the liquid on the prismatic spectrum. In Fig.
21 the curve represents the relative intensity of the coloured
light in different portions of the spectrum. If we cut off

successively slices of the rectangle, as is done in Figs. 23
4

72 MODERN CHROMATICS.

A B C D E F G

RED

YEU

SREEH

BUIE.

VIOLET

Fig. 21. — ^The sliaded portion represents the amount of light transmitted by chloride of

chromium.

and 23, we obtain the curves corresponding to a greater
and greater thickness of liquid, and it is plain that at last
we shall have the state of things indicated in Fig. 23 ; the

ABC

RED YEL. GREEN BLUE VIOLET

Fig. 22. — Chloride of Chromium; Effect produced by a Thick Layer.

curve is about the same as for red glass (Pig. 17), and the
final colour is red. This is an extreme case, but in stained
glasses, pigments, dyestuffs, etc., there is generally a ten-

A B C

RED YEL. GREEN BLUE VIOLET

Fia. 28.— Chloride of Chromium ; Kfiect produced by a very Thick Layer.

dency toward the production of effects of this kind, some
of which will hereafter be noticed.

The colours of painted glass are similar to those of
stained glass in origin and properties ; both are intense,
rather free from admixture with white light, and capable

ON THE PRODUCTION OF COLOUR BY ABSORPTION. 73

of a high degree of luminosity. In these respects they far
surpass the colours of pigments, which compared with them
appear feeble and dull, or pale. Oiying to this circum-
stance, chromatic combinations may be successfully worked
out in stained glass, which would prove failures if attempt-
ed with pigments or dyestuflEs. Hence also the wonderfully
luminous appearance of paintings on glass viewed in a prop-
erly darkened room : they surpass in some respects oil or
water-colour paintings to such a degree that the two are not
to be mentioned together. There is no doubt but that
glass-painting offers advantages for the production of real-
istic eflEects of colour and light and shade, such as the very
narrow scale of oil and water-colour utterly denies ; and
yet great artists seem to reject this process, and severely
confine themselves to work on canvas or paper, choosing to
depend for their effects rather on pure technical skiU and
artistic feeling.

If we place on a sheet of white paper a fragment of pale-
blue glass, it will display its colour, though not so brilliantly
as when held so that the light of the window streams through
it directly. The reason is very evident : the light which
penetrates the glass falls on the paper and is reflected by it
back through the glass to the eye. The light then traverses
the glass twice, but this is not the only cause of its inferior
luminosity, for a double plate of the same glass held before
the window appears still far brighter than the single glass
on the paper. The other reason is that the paper itself re-
flects only a small amount of the light falling on it. Upon
examining the matter more closely we find also that the blue
glass reflects from its surface quite a quantity of white light,
which, when mingled with the coloured light, renders it
somewhat pale. If, now, we grind up into a very fine pow-
der some of the blue glass, we obtain the pigment known
as smalt, and, after mixing it with water, we can wash our
white paper with a thin layer of it. When it dries the

74 MODERN CHROMATICS.

paper will be coloured blue, but the hue will be neither so
luminous nor so intense as that of the light directly trans-
mitted by the blue glass when held before a window. Its
origin, however, is similar : the white light after traversing
a layer of the minute blue particles reaches the paper, and
is reflected backward once more through them toward the
eye. In this process many coloured rays suffer absorption,
and only a small portion of the constituents of the original
beam finally reach the eye. In the original experiment,
where the blue glass was simply laid on the white paper, it
sometimes happened that the white light regularly reflected
from its first surface mingled itself with the coloured light
and caused it to look paler, but it was always possible to
arrange matters so that this damaging coincidence did not
occur.- In the experiment with the blue powder spread on
the paper this is impossible, for the surfaces of the little
particles lie with all possible inclinations, so that, hold the
paper as we will, it is sure to reflect much white along with
its coloured light. What we have, then, to expect when
pigmeiltB in dry powder are spread on white paper is, that
they will reflect only a moderate quantity of colom'ed light
to the eye, and that it will be rendered somewhat pale by
admixture with white light.

With the aid of a little hand spectroscope these points
are readily demonstrated : when we direct the instrument
toward our blue paper, we find that all the colours of the
spectrum are present in considerable quantities — hence some
white light must be reflected from the paper ; we also notice
that the red, yellow, orange, and green rays are present in
less quantity than in an ordinary prismatic spectrum — Whence
the curve for the smalt-paper is like that given in Fig. 24.
In making examinations with the spectroscope of the col-
oured light reflected from painted surfaces, it is advanta-
geous to use simultaneously, along with the strip of painted
paper, one which is white and a third which is black. It
has been found by the author that paper painted dead-black

ON THE PRODUCTION OF COLOUR BY ABSORPTION. 75

with lampblack, to which has been added just enough spirit
varnish to prevent its rubbing off, but not enough to cause
it in the least degree to shine, reflects yf^ as much light as
white paper. Hence if we set the luminosity of white paper
as 100, that of dead-black paper will be 5. Now, when a

ABC

RED

Fio. 24.—

,fl??^^^

'/hmv^/'m/////wr/i/,

y£L.

Gfl££N

BUit

VIOLZT

Curve for Bmalt-paper ; the shaded portion represents the light reflected
by smalt-paper.

strip of this black paper is placed before the slit of the spec-
troscope it acts like white paper seen under a feeble illumi-
nation, and consequently furnishes a complete though not a
very luminous spectrum. By using, then, a black-and-white
strip along with the one which has been painted, we can
ascertain several facts which may best be explained with
the help of an example. Let us first select vermilion in dry
powder, and undertake an examination of its optical proper-
ties in this way. We find that the red of its spectrum is
about as powerful as the red in the spectrum from white
paper, and that the other colours, though all present, are
not much if any stronger than those from the black paper.
This is all we can demand from any pigment : it reflects to
the eye its full share of the rays it professes to reflect, and
they are not mingled with more white light than is reflected
by dead-black paper. Emerald-green when tested in this
way proves sensibly inferior to vermilion : examined in
dry powder the green space was bright, but less bright than
that from white paper ; the other colours had about the
same degree of luminosity as those from the black paper,

76 MODERN CHROMATICS.

except the violet, which was not present. Chrome-yellow
reflected the red, orange, yellow, and green rays about as
brilliantly as white paper ; the cyan-blue, ultramarine, and
violet, about like black paper. Hence the great luminosity
of this pigment, for it reflects not only the yellow rays
abundantly, but also all the other rays of the spectrum
which are distinguished for luminosity. As before re-
marked, the sum of these rays makes up yellow. It is plain
from these experiments that a painted surface can never be
as luminous as one which is white ; the most that can be
demanded from a painted surface is, that it should reflect
its peculiar coloured light as powerfully as a white surface
does ; the very cause of its furnishing coloured light is, that
it fails to reflect all the coloured rays equally well. Hence
coloured surfaces are always darker than those which are
white. If we set the luminosity of white paper as 100, that
of vermilion will be about 25, emerald-green 48, and chrome-
yellow as high as 75 or 80.

These experiments can now be repeated with the same
pigments covered by a layer of water. The surface of the
water being quite flat, the spectroscope can be held in such
a way as to avoid the light directly reflected from the water,
and it then becomes possible to observe certain changes
which the presence of the water brings about. In the case
of vermilion we find that the blue and violet portions of the
spectrum almost entirely vanish, a little of the yellow,
orange, and green spaces remains, and the red is nearly as
powerful as before. This proves that the presence of the
water has greatly diminished the amount of white light re-
flected from the surfaces of the particles of pigment, but
has not much affected the brilliancy of the reflected col-
oured light. Experiments with emerald-green and chrome-
yellow give corresponding results ; less light in general is
reflected, but it is somewhat purer, there being not so much
white light mingled with it. ■ By immersing our pigments
in oil or varnish we push these effects still further : the

ON THE PRODUCTION OF COLOUE BY ABSORPTION. 77

pigments appear darker, but the colour is richer, and more
nearly free from "white light. The explanation of these
changes is well known to physicists : they depend upon the
fact that light moving in a rare medium like the air is abun-
dantly reflected when it strikes on a dense substance like a
pigment ; but if the pigment be placed under water we
have then light moving in a dense medium (water), and
striking on one which is only a little more dense (pigment) :
hence but little white light will be reflected from the sur-
face of the small particles. The coloured light which is so
abundantly furnished by the pigment, even under water,
has its source in reflections which take place in the interior
of the somewhat coarsely grained particles of the pigment
itself. If the pigment is naturally fine-grained, and also is
mixed with a liquid like oil, having about the same optical
density as itself, scarcely any light will be reflected from it,
coloured or otherwise. Prussian-blue and crimson-lake,
ground in oil, are good examples. In order to exhibit their
colours it is necessary either to spread them in thin layers
over a light surface, or to mix them with a white pigment ;
alone by themselves they appear very dark, the Prussian-
blue, indeed, almost black. Many other pigments are more
or less affected in the same way by the presence of oil or
varnish.

From what has been stated above it follows that the
medium with which pigments are mixed has an important
influence on their appearance. In drawings executed in
coloured chalks, and in oU-paintings, we have the two ex-
tremes, works in water-colour being intermediate. Hence
oil-painting is characterized by the richness of the colouring
and the transparency and depth of its shadows, while in
pastel drawings the tints are paler, the shadows less intense,
and over the whole is spread a soft haze which lends itself
readily to the accurate imitation of skies and distances.
Changes in the medium are sometimes a source of embar-
rassment to the painter. This is particularly true in the

78 MODERN CHROMATICS.

process of fresco-painting, and also to some extent in that
of water-colour : as long as the pigment is moist it appears
darker than afterward when dry, and it is necessary for the
artist in laying on each wash to make a proper allowance
for these changes ; this is one of the minor causes that ren-
der the process of painting in water-colours tnore difficult
than that in oils.

As has already been stated, when we obtain our coloured
light from pigments, it is apt to be more mingled with white
light than when stained glass is used ; but, besides this, it
is inferior to that from stained glass in the matter of lumi-
nosity. The range of illumination in our houses is small,
so that practically the scale of light at the disposal of the
painter in oils or water-colours is quite limited ; in point of
fact he is obliged by the necessities of the case to employ
means which are quite inadequate : hence the extraordinary
care with which he husbands his resom-ces in the matter of
light and shade, and his constant struggle for excellence
and decision in colouring. Muddy and dirty colours are
instantly recognized to be such under a feeble illumination,
even though they have passed muster under the blaze of
full sunlight. Almost any surface looks beautiful if very
brightly illuminated ; the eye is dazzled, and remains un-
conscious of defects that are instantly exposed under the
feebler light of a gallery.

The colours which are exhibited by woven fabrics are
due, like those of stained glass, to absorption. In the case
of silk and wool the dye penetrates the fibres through and
through, so that under the microscope they have much the
same appearance as fine threads of stained glass. When
white light falls upon a bundle of such coloiu*ed fibres, a
portion is reflected uncoloured from the surface of the top-
most fibres, while another portion penetrates to the rear
surfaces of these same fibres and there is again subdivided,
some rays penetrating still deeper into the bundle, while
others returning to the upper surface emerge coloured.

ON THE PRODUCTION OF COLOUR BY ABSORPTION. 79

This process is repeated on each deeper-lying set of fibres,
and the result is that a good deal of strongly coloured light
is sent to the eye, mingled with a portion due to the surface
layers, which is more faintly coloured ; there is in addition
a small portion which is quite white. It will be seen that
the reflective power of the fibres is an important element in
this process, for all the coloured light which reaches the eye
is sent there by reflection. If we take similar structures of
silk and wool, we can compare directly the lustre or reflective
power of the individual fibres, with the aid of a lens mag-
nifying ten or fifteen diameters. A silk-cocoon and a piece
of white felting answer very well for this pui-pose, and
when they are compared under the microscope it is very
evident that the natural lustre of the silk is greatly superior
to that of the wool. On comparing in this way the felting
with white cotton batting, it will be found that the wool
surpasses the cotton in lustre, the latter appearing almost
dead-white and free from sparkle. It follows from this that
the coloured light which is reflected from sUk is more satu-
rated or intense, and appears richer, than that from wool.
The fibres of silk also can be made to lie in straight, paral-
lel, compact bundles, which enables them to reflect the white
light in definite directions, whereas woollen fabrics reflect it
equally well in all directions. It results from this that a
fabric of silk is capable, according to circumstances, of ex-
hibiting a rich saturated colour nearly free from white
light, or it may reflect much white light and exhibit a pale
colour. This sparkling play of colour is beautiful, and
causes the more uniform appearance shown by woollen
fabrics to appear dull and tame. On the other hand, the
superior transparency of the dyed fibres of wool over those
of cotton give to the colours of the former material a cer-
tain appearance of richness and saturation, and cause the
tints of the cotton to appear somewhat opaque.

In velvet the attempt is made to suppress all surface-light,
and to display only those rays which have penetrated deeply

80 MODERN CHROMATICS.

among the fibres, and have become highly coloured. This is
accomplished by presenting to the light a surface which is
entirely composed of the ends of fibres, and consequently
which has little or no capacity for reflecting light. The
rays then penetrate between the fibres thus set up on end,
and, after wandering among them, finally again in some
small quantity reach the surface as richly coloured light,
which produces its full effect undiminished by any admix-
ture of white light from the surface. In the case of sUk-
velvet the desired effect is for the most part actually real-
ized : the colours are rich, and an examination with a lens
shows that scarcely any of the fibres reflect white light,
even when the fabric is held in unfavourable positions. If
cotton-velvet is subjected to a similar examination under a
lens, it will be found to reflect much surface-light, particu-
larly when not quite new, and the surface will present a
broken, rough appearance, quite different from that of its '
more aristocratic rival.

It would appear that at present it is actually possible to
employ for woven fabrics dyes which furnish coloured light
having a degree of intensity and purity which is actually
undesirable. This is the case ynth some of the aniline dyes.
Dresses dyed with some of them, when seen in full daylight,
act on the eye so powerfully that mere momentary inspec-
tion gives rise to the phenomenon of accidental colours (see
Chapter VIII.). These harsh effects are interesting as con-
veying certain information that our dyers have already
touched, and indeed gone beyond, the greatest allowable
limits in the matter of the intensity and purity of then-
hues. At least this applies to large surfaces, such as com-
plete dresses, etc. In the case of smaller articles, such as
ribbons, etc., these intense colours are more allowable, just
as the flash of diamonds is more tolerable on account of
their insignificant size.

We have seen, thus far, that the colours of pigments
and dyestuffs are due to absorption, and to this same cause

ON THE PRODUCTION OF COLOUR BY ABSORPTION. 81

we must attribute the colours of most objects which occur
in landscapes. Two of these are so important that it will
be worth while to devote a few moments to their separate
consideration : we refer to the colour of water, and to that
of vegetation. The colour of large masses of water, such
as lakes and rivers, is so much influenced by that of the
sky that many persons consider it to depend wholly on it,
and are disposed to doubt whether water has any proper
colour of its own. It is quite true that a small quantity of
pure water, such as is contained in a drinking-glass, appears
perfectly colourless, and that the light from white objects
passes through it without suffering sensible absorption. K,
however, we allow the white light from a porcelain plate to
traverse a layer of pure distilled water two metres in thick-
ness, it will be found to be tinged bluish. This experiment,
which was first made by Bunsen, proves that an absorption
takes place along the red end of the spectrum, and that
water is really coloured in the same sense as a weak solution
of indigo. The water of the lake of Geneva is quite pure,
being produced mainly by the melting of glaciers ; the
granite meal mingled with the water, being coarse, soon
settles to the bottom, and leaves it free from turbidity.
Hence along the wonderful shores of this lake it is easy
to repeat the experiment of Bunsen, and to study the colour
of this liquid. White objects, resting on the bottom in the
shallow places where the depth is six or eight feet, show
very plainly a greenish-blue hue, and the tint can be exam-
ined at different depths by lowering a piece of white porce-
lain with a string. Even on cloudy days, when the sky is
overcast and grey, the lake itself displays a wonderfully in-
tense cyan-blue colour, while on clear days, on looking
down into its waters, one is tempted to believe that it
is a vast natural dyeing-vat. When vegetable matter is
present in small quantity the colour of water changes to a
bluish-green ; many excellent examples occur among the
beautiful lakes of the Tyrol. Decaying organic matter,

MODERN CHROMATICS.

on the other hand, tinges water brownish, and lakes or
rivers of this colour are apt to assume on cloudy days a
silver-grey appearance, while under a clear sky they often
appear very decidedly blue. There seems to be some reason
to believe that the absorptive action of pure water on white
light changes with its temperature, and that warm water is
actually more deeply coloured than cold water. Heat has
an action of this kind upon many coloured substances, and
Wild with his photometer actually found that both distilled
and pump water showed somewhat stronger colours on
being heated. He accounts in this way for the niore in-
tense colour which it is claimed mountain lakes display
during the summer months.

The green colour of vegetation offers a rather peculiar
. case. When we examine with the spectroscope any ordi-
nary green pigment, we find that the red is absent and the
blue and violet much weakened, as was the case with em-

A B CPE

SED YEL. GREEN BLUE VIOLEF

Fig. 25.-1116 shaded portioTi represents the light reflected by green leaves.

erald-green. Green leaves, however, furnish a spectrum of
a different character : the extreme red is present ; then
occurs a deficiency of coloured light, which is followed by
ah orange-red space ; next comes the orange, then the yel-
low, greenish-yellow, and yellowish-green ; . after this fol-
lows a little full green ; the rest of the spectrum decreases
rapidly in luminosity. Fig. 25 represents this spectrum.
The sum of all these colours is a somewhat yellowish green,
which is accordingly the colour presented by green leaves

ON THE PRODUCTION OF COLOUR BY ABSORPTION. 83

in white light. It will be shown in Chapter X. that a mix-
ture of red and green light furnishes yellow light, which
explains the production of a yellowish-green in this some-
what singular way. It follows, from the analysis just given,
that green leaves are capable of reflecting a considerable
quantity of red Kght, where surfaces painted with green
pigments would not have this power, and conseqpiently
would appear black or grey. Hence under the red light of
the setting sun foliage may assume a red or orange-red hue.
Corresponding to this, when the illumination is of an orange
colour, foliage will partake more of this hue than would be
the case with ordinary -green pigments. Connected with
this is also the great change of colour which foliage expe-
riences according as it is illuminated by direct sunlight or
by light from the blue sky, the tint in extreme cases varying '
from a yellow or slightly greenish yellow up to a bluish-green.
Simler has constructed a simple and beautiful piece
of apparatus, based on the singular property which living
leaves have of reflecting abundantly the extreme red rays
of the spectrum ; it is called an erythroscope. A plate of
blue glass, stained with cobalt, is to be procured, having a
thickness such that it will allow the extreme red of the
spectrum to pass, but no orange or yellow ; it should also
transmit the small band of greenish-yellow just before the
fixed line E, and all the green from b to F> also all the blue
and violet. A plate of rather deeply coloured yellow glass
is also needed ; this should be capable of transmitting all
the light of the spectrum from the farthest red up to G ;
that is to say, it should cut off the violet and blue-violet
only. When a sunny landscape is viewed through these
two glasses, it assumes a most wonderful appearance : all
green trees and plants shine with a coral-red colour, as
though they were self-luminous ; the sky is cyan-blue,* the
clouds purplish-violet ; the earth and rocks assume various

* Cyan-bluc is a greenish-blue.

84 MODEEN CHROMATICS.

tints of violet-grey. Pine-trees appear of a dark-red hue ;
orange or yellow flowers become red or blood-red ; greens,
other than those of the foliage, are seen in their natural
tints, or at least only a little more bluish ; lakes preserve
their fine blue-green colouring, and the play of light and
shade over the landscape is left undisturbed ; the whole
effect is as though a magician's -wand had passed over the
scene, and transformed it into an enchanted garden. For
the full realization of these effects it is essential that stray
light should be prevented from reaching the eyes, and ac-
cordingly the glasses should be mounted in an arrangement
of wood or pasteboard -which adapts itself to the contours
of the face, and excludes as much as possible diffuse light.
On comparing the spectrum given by the blue and yellow
glasses with that of green leaves, it will be found that the
two glasses cut off almost all the green light furnished by
the leaves, but allow those green rays of light to pass which
the leaves are incapable of supplying.

The colours which metals such as copper, brass, or gold
display, are due to absorption. A quantity of white light
is reflected from the real surface, but along with it is min-
gled a certain amount which has penetrated a little distance
into the substance of the metal, and there has undergone
reflection ; this last portion is coloured. If we cause this
mixture of white and coloured light to strike repeatedly on
a metallic surface — for example, such as gold — we constant-
ly increase the proportion of light which has penetrated
under the surface, and has become coloured. A process of
this kind takes place in the interior of a golden goblet ;
hence the colour in the inside is deeper and more saturated
than on the outside. Some metals, like silver or steel, hard-
ly show much colour till the light has been made to strike
repeatedly on their surfaces ; when this is done with sUver,
the light gradually assumes a yellow colour, while with
steel it becomes blue.

ON THE PRODUCnON OF COLOUR BT ABSORPTION. 85

The true colour of metals must not be confounded with
that which is often given to them by the presence of a sur-
face-film of oxide or sulphide ; such films cause for the
most part a bluish appearance, though all the colours of the
spectrum may be produced on metals in this way. In fact,
the. hue in these cases is due to an interference of light
caused by the thin layer of oxide, and is quite distinct from
the actual colour of the metal. (See Chapter IV.)

Metals, whether coloured or white, are chiefly remark-
able for the large quantities of light which they are capable
of reflecting. Measurements made by Lambert have shown
that the total amount of light reflected by white paper is
about forty per cent, of the light falling on it. Silver,
however, is capable of reflecting ninety-two per cent. ; steel
sixty per cent., etc.

Polished surfaces, particularly of metals, have another
property which adds to their apparent brilliancy, and in-
creases their lustrous appearance. Those portions of the
surface which are turned away from the light often reflect
but little, and look almost black. This sharp contrast en-
hances their brilliant, sparkling appearance, and raises them
quite above the rank of surfaces coloured by pigments. In
consequence of this, metals cannot be used along with pig-
ments in serious or realistic painting ; they are quite out of
harmony, and produce the impression that the painter has
sought to help himself by a cheap trick rather than by em-
ploying the true resources of art. In those cases where
gold was so extensively used during the middle ages for
the backgrounds of pictures of holy personages, or even for
the adornment of their garments, the object was far more
to produce symbolic than realistic representations, and here
the presence of the gold was actually a help, as tending to
convey the idea that the painting was not the portrait of an
ordinary mortal, but rather a childlike attempt to depict
and lavishly adorn the ideal image of a venerated and
saintly character. On the other hand, this brilliancy of

86 MODERN CHROMATICS.

gold, with its rich colour, preeminently adapts it for the
purposes of inclosing paintings and isolating them from sur-
rounding objects. A painted frame or wooden frame, inas-
much as its colour belongs to the same order as those con-
tained in the picture, becomes as it were an extension of it,
and is apt to injure the harmony of its colouring ; and, be-
sides this, its power of isolation is inferior to that of gold,
on account of its greater resemblance to ordinary surround-
ing objects.

Having now considered with some detail the colours
that are produced by absorption, it may be well to add a
few words concerning the attempts that have been made to
reproduce colour by the aid of photography. Photographs
render accurately the light and shade, why should they not
also record the colours, of natural objects ? In 1848 E.
Becquerel announced that he had been able to photograph
the colours of a prismatic spectrum falling on a silver plate
which had been treated with chlorine. These colors were
quite fugitive, lasting only a few minutes. In 1850 Nifepce
de Saint- Victor and in 1852 J. Campbell claimed that they
had rendered these colours more permanent. In 1863 the
former experimenter, by washing the finished plates with a
solution of dextrin containing chloride of lead, obtained
coloured pictures that lasted twelve hours. In the following
year he still further improved his process, the colours last-
ing three or four days in rather strong daylight. An ex-
amination of the details of these memoirs and of the pic-
tures indicates that the colours thus obtained are due to a
greater or less reduction of the film of chloride of silver,
and are, in fact, produced merely by the interference of
light, and consequently have no necessary connection with
the hues of the natural objects to which they seem to owe
their origin. Hence we must regard this problem as un-
solved, and in the present state of our knowledge there'

ON THE PRODUCTION OF COLOUR BY ABSORPTION. 87

does not seem to be any reason to suppose that it ever will
be solved. Why should the red rays when acting on a cer-
tain substance produce a red compound, the green and vio-
let rays green and violet compounds, and so on with all the
other coloured rays ? But photography in colour implies
exactly this.

This problem has more recently been handled in an en-
tirely different manner, and with a more hopeful result,
from a practical point. of view. Suppose we place a red
glass before a photographic camera, and photograph some
object with brilliant colours — a carpet, for example. We
shall obtain an ordinary negative picture, which will be en-
tirely due to the red light sent by the carpet toward the
instrument. Portions of the carpet having a different col-
our will not be photographed at all. Next let us hold be-
fore the camera a glass which transmits only the yellow
rays (if such glass could be found), and we shall obtain a
picture of the yellow constituents of the carpet ; the same
is to be done with a blue glass. From these three ordinary
negatives, three positive pictures are to be made in gelatine,
the first being colored with a transparent red pigment, the
second with a yellow, the third with a blue pigment. The
first sheet of gelatin will contain a red picture, due to the
red parts of the carpet ; the second and third, similar yel-
low and blue pictures. When these transparent coloured
sheets are laid over each other, we shall have a picture cor-
rect in drawing, which will roughly reproduce the cplom-s
of the carpet. This gives an idea of the plan proposed in
1869 by C. Cross and Ducos du Hauron, for the indirect
reproduction of colour by photography. In actual practice
the negatives were taken with glasses coloured orange,
green, and violet ; these negatives were then made to yield
blue, red, and yellow positive pictures. This process has
been greatly improved by Albert, of Munich, and by Bier-
stadt, of New York. In the final picture the gelatine is dis-
pensed with, films of colour, laid on by lithographic stones,

MODERN CHROMATICS.

being substituted. The selection of the pigments is neces-
sarily left to the judgment of the operator, and in its pres-
ent state the process is better capable of dealing with the
decided colours of designs made by the decorator than with
the pale, evanescent tints of Nature.

APPENDIX TO CHAPTER VII.

We give below a list of pigtoents whioli, according to Field and
Linton, are not affected by the prolonged action of light, or by fonl
air:

WliUe.
Zinc-white.
True pearl-white.
Baryta-white.
Tin-white.

Sed.

Vermilion.
Indian red.
Venetian red.
Light red.
Red ochre.

Yellow.

Cadmium-yeilow.
Lemon-yellow.
Strontia-yellow.
Yellow ochre.
Raw Sienna.
Oxford ochre.
Roman ochre.
Stone ochre.
Brown ochre.

Illach.
Ivory-black.
Lampblack.
Indian ink.
Graphite.

Orange vermilion.
Jaune de Mars.
Orange ochre.
Burnt Sienna.
Burnt Roman ochre.

Green.
Oxide of chromium.
Rinman's green.
Terre-verte.

Mile.
Ultramarine.
Blue ochre.

Violet.
Purple pchre.
Violet de Mars.

Brown.
Rubens's brown.
Vandyck brown.
Raw umber.
Burnt umber.
Cassel earth.
Cologne earth.
Bistre.
Sepia.
Asphalt.

APPENDIX TO CHAPTEK VIL

White lead, smalt and cobalt-Wue ave not affected by light, but
are by foul air. The last two are considered permanent in water-
colour painting.

According to Field, the tints of the following pigments are not
affected by mixture with lime, consequently they are adapted for
use in fresco-painting :

White.

Baryta.
Pearl.
Gypaiim.
Pure earths.

Sed.

Vermilion.
Red lead.
Red ochre.
Light red.
Venetian red.
Indian red.
Madder red.

Yellow.

Indian yellow.
Yellow ochre.
Oxford ochre.
Roman ocbre.
Stone ochre.
Brown ochre.
Raw Sienna.
Naples yellow.

Black.

Ivory-black.
Lampblack.
Black chalk.
Graphite.

Orange.
Orange vermilion.
Chrome-orange.
Orange ochre.
Jaune de Mars.
Burnt Sienna.

Green.
Terre verte.
Emerald green.
Mountain green.
Cobalt-green.
Chrome-green.

Blric.
Ultramarine.
Smalt.
Cobalt.

Fwple.
Madder purple.
Purple ochre.

Brown.
Vandyck brown.
Bubens's brown.
Raw umber.
Burnt umber.
Cassel earth.
Cologne earth.
Antwerp brown.
-Bistre.

As the effect of light on pigments is a matter of considerable
importance to artists, particularly to those working with the thin
washes used in water-colour painting, a careful experiment on this

90 MODERN CHROMATICS.

matter was made by the present writer. The washes laid on or-
dinary drawing-paper were exposed during the summer to sunlight
for more than three months and a half, and the effects noted ; these
were as follows :

Watek-colouk Pigments that are not affected by Light:

Red.

Orange.

Yellow.

Indian red.

Jaune de Mars.

Cadmium-yellow.

Light red.

Yellow ochre.
Roman ochre.

(h-een.

Blue.

Brown.

Terre verte.

Cobalt.

Burnt umber.

French blue.

Burnt Sienna.

Smalt.

New blue.

The following pigments were all more or less affected ; those
that were very little changed head the list, which is arranged so as
to indicate the relative amounts of damage suffered, the most fugi-
tive colours being placed at its end :

Chrome-yellow becomes slightly greenish.

Red lead becomes slightly less orange.

Naples yellow becomes slightly greenish brown. •

Raw Sienna fades slightly ; becomes more yellowish.

Vermilion becomes darker and brownish.

Aureoline fades slightly.

Indian yellow fades slightly.

Antwerp blue fades slightly.

Emerald green fades slightly.

Olive green fades slightly, becomes more brownish.

Rose madder fades slightly, becomes more purplish.

Sepia fades slightly.

Prussian blue fades somewhat.

Hooker's green becomes more bluish.

Gamboge fades and becomes more grey.

Bistre fades and becomes more grey.

Burnt madder fades sSmewhat.

Neutral tint fades somewhat.

Vandyok brown fades and becomes more grey.

Indigo fades somewhat.

Brown pink fades greatly.

APPENDIX TO CHAPTER VII. 91

Violet carmine fades greatly, becomes brownish.
Yellow lake fades greatly, becomes brownish.
Crimson lake fades out.
Carmine fades out.

To this we may add that rose madder, burnt or brown madder,
and purple madder, all, are a little affected by an exposure to sun-
light for seventy hours. Pale washes of the following pigments
were completely faded out hy a much shorter exposure to sunlight :

Carmine, Yellow lake, Italian pink.

Full red. Gall-stone, Violet carmine.

Dragon's blood. Brown pink.

CHAPTER VIII.

ON THE ABNORMAL PERCEPTION OF COLOUR, AND
ON COLOUR-BLINDNESS.

We have considered now, with some detail, the various
ordinary modes of producing the sensation of colour ; but,
in order to render our account more complete, it is necessary
to touch on some of the unusual or extraordinary methods.
In every case examined thus far, the sensation of colour was
generated by the action on the eye of coloured light — that
is, of waves of light having practically a definite length.
As colour, however, is only a sensation, and has no existence
apart from the nervous organization of living beings, it may
not seem strange to find that it can be produced by white
as well as by coloured light, or even that it can be developed
in total darkness, without the agency of light of any kind
whatever. If the eyes be directed for a few m.oments to-
ward a sheet of white paper placed on a black background
and illuminated by sunlight, on closing them and excluding
all light by the hands or otherwise, it will be found that
the absence of the light does not at once cause the image
of the paper to disappear. After the eyes are closed it will
stiU be plainly visible for several seconds, and will at first
be seen quite correctly, as a white object on a black ground ;
the colour with some observers then changes to blue, green,
red, and finally back to blue, the background remaining all
the while black. After this first stage the background
changes to white, the colour of the sheet of paper appearing
blue-green, and finally yellow. Most of these colours are

ON THE ABNORMAL PERCEPTION OF COLOUR, ETC. 93

as distinct and decided as those of natural objects. If the
experiment be made for a shorter time, and under a less
brilliant illumination, the eyes being first well rested by-
prolonged closure, the series of colours will be somewhat
different. Fechner, Seguin, and Helmholtz observed that
the original white colour passed rapidly through a greenish
blue (Seguin, green). into a beautiful indigo-blue ; this af-
terward changed into a violet or rose tint. These colours
were bright and clear, afterward followed a dirty or grey
orange ; during the presence of this colour the background
changed from black to white, and the orange tint altered
often into a dirty yellow-green which completed the series.
If, instead of employing white, a coloured object on a grey
ground is regarded intently for some time, the eyes will be
so affected that, on suddenly removing the coloured object,
the grey ground will appear tinged with a complementary

Fio. 26.— Disk with Black and Fig. 27.— Black and White Spiral

White Sectors for the Produo- on Disk, for the Production of

tion of Subjective Colour. Subjective Colour.

tint ; for example, if the object bo red, the after-image will
be bluish green. It is not necessary to dwell longer on
these phenomena at present, as a portion of Chapter XV.
will be especially devoted to them. In both the cases men-
tioned above, the colour develops itself after the eyes are.^.
closed, or at least withdrawn from the illuminated surface. ■
There are, however, cases where very vivid colours can be
seen while the eyes are exposed to full daylight. If a disk

MODERN CHEOMATICS.

of cardboard painted witli alternate white and black sectors,
like that shown in Fig. 26, be set in rotation while exposed
to full daylight, colours will be seen after a few trials. It
will be found that a certaui rate of rotation communicates
to the disk a green hue, a somewhat more rapid rate causing
it to assume a rose colour. According to Helmholtz, these
effects are most easily attained by using a disk painted with
a black spiral, like that in Fig. 27. These phenomena may
be 9.dvantageously studied by a method which was used by
the author several years ago. A "blackened disk with four
open sectors seven degrees in width was set in revolution
by clockwork, and a clouded sky viewed through it. With

Fig. 28.— Subjective Colours
seen in Sky, with aid of

Kotating Bisk.

Fia. 29.— Subjective Colours, Kinj,
etc., seen in Sky witli aid of Eo-
tating Disk.

a rate of nine revolutions per second, the whole sky often
appeared of a deep crimson hue, except a small spot in the
centre of the field of view, which was pretty constantly yel-
low. Upon increasing the velocity to eleven and a half
revolutions per second, the central spot enlarged somewhat,
and became coloured bluish green, with a narrow, faint, blue
border, indicated by the dotted line ; the rest of the sky
appeared purple, or reddish purple. (See Fig. 28.) With
the exception of fluctuation's in the outline of the spot, this
appearance remained tolerably constant as long as the rate
of revolution was steadily maintained. When the velocity

ON THE ABNORMAL PERCEPTION OP COLOmi, ETC.

of the disk was increased, the bluish-green spot expanded
into an irregularly shaped blue-green ring, which with a
rate of fifteen turns per second mostly filled the whole field
of view. (See Fig. 29.) With higher .rates all these ap-
pearances vanished, and the sky was seen as with the naked
eye.* More than one elaborate attempt has been made to
found on phenomena of this class a theory of the production
of colour, though it may easily be shown that in all such
cases the disk really transmits not coloured but white light,
and that the effects produced are due to an abnormal state
of the retina caused by alternate exposure to light and
darkness.

A current of electricity is also capable of stimulating
the optic nerve in such a way that brilliant colours are per-
ceived, although the experiment is made in perfect dark-
ness. If the cm-rent of a strong voltaic battery be caused
to enter the forehead, and travel hence to the hand, accord-
ing to Ritter, a bright-green or bright-blue colour is per-
ceived, the hue depending on whether the positive cm-rent
enters the hand or forehead. Helmholtz, in repeating this
operation, was conscious simply of a wild rush of colours
without order. The experiment is, however, interesting to
us, as proving the possibility of the production of the sen-
sation of colour without the presence or action of light.

Recently a substance has been discovered which, when
swallowed, causes white objects to appear coloured greenish
yellow, and coloured objects to assume new hues. Persons
under the influence of santonin cannot sec the violet end of
the spectrum ; and this fact, with others, proves that they
have become temporarily colour-blind to violet.

An observation of Tait's, and others by the author, have
shown that a shock of the nervous system may produce
momentarily colour-blindness to green light. White objects
then appear of a purplish red, and green objects of a much

• "American Journal of Science and Arts," September, 1860.
5

96 MODERN CHROMATICS.

duller green hue than ordinarily.* These effects are eva-
nescent, though quite interesting, as we shall see presently,
from a theoretical point of view.

Investigations during the present century have shown
that many persons are born with a deficient perception of
colour. In some the defect is slight and hardly noticeable,
while in others it is so serious as to lead to quite wonderful
blunders. This imperfection -of vision is often inherited
from a parent, and may be shared by several members of
the same family. It is remarkable that women are com-
paratively free from it, even when belonging to families of
which the male members are thus affected. The occupations
of women, their attention to dress and to various kinds of
handiwork where colour enters in as an important element,
seem to have brought their sense for colour to a higher de-
gree of perfection than is the case with men, who ordinarily
neglect cultivation in this direction. Out of forty-one
young men in a gymnasium, Seebeck found five who were
colour-blind ; but during his whole investigation he was
able to learn of only a single case where a woman was to
some extent similarly affected. It not unf requently happens
that persons with this defect remain for years unconscious
of it. This was the case with some of the young men in-
vestigated by Seebeck ; and in one remarkable instance a
bystander, in attempting to help a colour-blind person who
was under investigation, showed that he was himself colour-
blind, but belonged to another class ! The commonest case
is a deficient perception of red. Such persons make no dis-
tinction between rose-red and bluish-green. They see in
the spectrum only two colours, which they call yellow and
blue. Under the name yellow they include the red, orange,
yellow, and green spaces : the blue and violet they name,
with some correctness, blue. In the middle of the spectrum

* "American Journal of Science and Arts," January, ISY'?. A similar
observation by Charles Pierce was communicated to the author while this
work was going through the press.

ON THE ABNORMAL PERCEPTION OF COLOUR, ETC. 97

there, is for them a neutral or grey zone, which has no
colour ; this, according to Preyer, is situated near the line
F. For the normal eye it is greenish-blue ; for them, white.
The extreme red of the spectrum, when it is faint, they fail
to distinguish ; the rest of the red space appears to them of
a saturated but not luminous green ; the yellow space has
for them a colour which we should call bright green ; and
finally, they see blue in the normal manner. Maxwell found
that by the aid of his disks, using only two colours, along
with white and black, he was able, by suitable variations in
their proportions, to match for them any colour which pre-
sented itself ; while the normal eye requires at least three
such coloured disks, besides white and black. His experi-
ments led to the result that persons of this class perceive
two of the three fundamental colours which are seen by the
normal eye. Helmholtz also arrived at the same result. It
is possible to render the nonnal eye to some extent colour-
blind to red in the manner followed by Seebeck in 1837, and
afterward by Maria Bokowa. These observers wore for
several hours spectacles provided with ruby-red glasses ;
and this prolonged action of the red light on the eye finally,
to a considerable extent, tired out the nerve fibrils destined
for the reception of red, so that on removing the glasses they
saw in the spectrum only two colours. The second observer
called them yellow apd blue. Furthermore, the extreme
red end of the spectrum was not visible to her, just as is the
case with those who are actually blind to red ; all red objects
appeared to her yellow, and dark red was not distinguishable
from dark green or brown.

Dalton, the celebrated English chemist, suffered from
this defect of vision, and was the first to give an accurate
description of it ; hence this affection is sometimes named
after him, Daltonism. It is very remarkable that, accord-
ing to the observations of Schelske and Helmholtz, even in
the normal eye there are portions which are naturally colour-
blind to red, and when this zone of the eye is used the same

98 MODERN CHROMATICS.

mistakes in matching colours are made. Such experiments
are somewhat difficult to make without considerahle prac-
tice, as it is necessary that the colored objects should be
viewed, not directly, but by the eye turned aside somewhat.
There is a simple means by which persons who are colour-
blind to red can to some extent help themselves, and prevent
the occurrence of coarse chromatic blunders, such as con-
fusing red with green. Green glass does not transmit red
light ; hence, on viewing green and red objects through a
plate of this glass it will be found, even by persons who are
colour-blind, that the red objects appear much more dark-
ened than those which are green. On the other hand, a red
glass will cause green objects to appear darker, but will not
affect the luminosity of those having a tint similar to itself.
The exact tints of the glasses are important, and they should
of course be selected with the aid of a normal eye.

The kind of colour-blindness just described is rather
common, and it has been estimated that in England about
one person in eighteen is more or less afflicted with it. We
pass on now to the consideration of a class of cases that is
more rare. Persons belonging to this second class see only
two colours in the spectrum, which they call red and blue.
They set the greatest luminosity in the spectrum in the
yellow space, as is done by the normal eye ; and they easily
distinguish between red and violet, but confuse green with
yellow and blue with red. In two cases examined by Preyer,
yellow appeared to them as a bright red ; this same observer
also found that in the spectrum, near the line h, the two
colours into which they divided the spectrum were separated
by a small neutral zone, which was for them identical with
grey. A sufficient number of observations have not been
accumulated to furnish means of ascertaining with certainty
the exact nature of the difficulty under which they labour,
though it is probable that they are colour-blind to green
light. There are also observations on record of cases of
temporary colour-blindness of a third kind, where the violet

ON THE ABNORMAL PERCEPTION OF COLOUR, ETC. 99

end of the spectrum was seen shortened to a very remark-
able extent ; and if it should prove that the cause was of a
nervous character, rather than due to a deeper yellow col-
ouration of the axial portions of the retina, this would
demonstrate the existence of violet colour-blindness.

The subject of colour-blindness is one of considerable
iniportance from a practical point of view, and this defect
has no doubt been the occasion of railroad accidents. In
1873-75 Dr. Favre, in France, examined one thousand and
fifty railroad officials of various grades, and found among
them ninety-eight pei-sons who were colour-blind — that is,
9"33 per cent. In 1876 Professor Holmgren, in Sweden,
examined the entire personnel of the Upsala-Gefle line, and
out of two hundred and sixty-six persons ascertained that
thirteen were colour-blind. These were found in every
grade of the service, many of them being required daUy to
make use of coloured signals. It is singular that in no case
nad there been previously any suspicion of the existence of
the defect. For further information with regard to the
practical side of this matter, the reader is referred to the
essay of Holmgren, which will be found in the Smithsonian
Report for 1877 : a French translation also exists.

In concluding this subject, it may not be amiss to allude
to the very remarkable case described by Huddart, of a
shoemaker, an intelligent man, where only a trace of the
power to distinguish colours seemed to remain.* According
to the observations, he was colour-blind to both red and
green, and in general seems to have had hardly any percep-
tion of colour, as distinguished from light and shade. Curi-
ously enough, recent observations of Woinow show that
even in the normal eye there is a condition like this at the
farthest limit of the visible field of view ; here all distinc-
tions of colour vanish, and objects look merely white or
black, or grey. It is probable that between the case of

* " Philosophical Transactions," Ixvii.

100 MODERN CHROMATICS.

Harris, just mentioned, and that of a normal eye possessed
of the maximum power of perceiving and distinguishing
colours, a great number of intermediate gradations will be
found to exist. Slight chromatic defects of yision generally
receive no attention, or are explained in some other way.
The writer recalls the case of an excellent physicist who
for many years had a half suspicion that he was to some
extent colour-blind, but rather preferred to explain the dis-
crepancies by the assumption of a difference in nomencla-
ture. Taking up the matter at last seriously, be made an
investigation of his own case, and found that he actually
was to some extent colour-blind to red. It has been sug-
gested that the very inferior colouring of some otherwise
great artists can be accounted for by supposing them to
have been affected with partial colour-blindness ; this idea
is plausible, but, as it appears to us, totally without proof.
There are great numbers of persons who are able to hear
distinctly all the notes employed in music, who yet have no
talent for it and no enjoyment of it. On the other hand,
we know of cases of persons who from infancy have been
afflicted with partial deafness, and have nevertheless been
musicians, and even composers. It is the same in painting
as in music : the possession of a perfect organ is not by any
means the first necessity, and it can be proved that even
artists who are actually colour-blind to red may still, with
but slight external aid, produce paintings which are univer-
sally prized for their beautiful colouring. Their field of
operations is of course more restricted, and they are com-
pelled to avoid certain chromatic combinations. During
the evening, by gas- or lamp-light, we are all somewhat in
the condition of persons who are colour-blind to violet ; but
yet, with precautions and some patience, it is possible to
execute works in colour, even at this time, which afterward
stand the test of daylight. It would appear probable, then,
that the difficulty with the inferior colourists above alluded
to was not so much anatomical or physiological as psychical.

ON THE ABNORMAL PERCEPTION OF COLOUR, ETC. 101

According to a theory recently proposed by Hugo Mag-
nus, our sense for colour has been developed during the last
four or five thousand years ; previous to this period it is
assumed that our race was endowed only with a perception
of light and shade. The evidence which is offered is of a
philological character, and tends to show that the ancients
either distinguished or described colours less accurately
than the modems. The same kind of reasoning might be
applied to proving that children at the present day have
but little power of distinguishing tints, as they usually
scarcely notice any but the most intense colours. When,
however, we study the prehistoric races at present existing
on the globe, and living in the same style as their ancestors,
we find them quite capable of discriminating colours, and
often very fond of them. Going many steps lower, we
meet with animals that have the power of perceiving and
even imitating a series of colours with accuracy. This is
the case with the chameleon, as shown by P. Bert, and also,
according to Pouchet and A. Agassiz, with certain kinds of
flounders. The skin of the chameleon is provided with an
immense number of minute sacs filled with red, yellow, and
black liquids ; the animal has the power of distending these
star-like vesicles at pleasure, and thus adjusts its colour in
a few minutes (after a series of trials) so as to match that
of the surface on which it is placed. Its chromatic scale
covers red, orange, yellow, and olive-green, and the mix-
tures of these colours with black, which includes of course
an extensive series of browns. The olive-green is made by
distending the yellow and black sacs, the effect being simi-
lar to that obtained by combining a black and yellow disk. .
(See Chapter XII.)

Corresponding to this, A. Agassiz has often noticed,
when a young flounder was transferred from a jar imitating
in colour a sandy bottom to one of- a dark-chocolate hue,
that in less than ten minutes the black-pigment cells would
obtain a great preponderance, and cause it to appear wholly

102 MODERN CHROMATICS.

unlike the yellowish-grey speckled creature which a few
moments before had so perfectly simulated the appearance
of sand. When removed to a gravelly bottom, the spots
on the side became prominent.

If our sense for light and shade is old, but that for
colour recent and still undergoing development, we should
perhaps expect that it would require more time to recog-
nize colour than appearances dependent simply on light
and shade ; but, according to the experiments of the writer,
forty billionths of a second answers as well for one as for
the other act of perception.*

In closing this chapter, it may be well, to mention a very
simple but beautiful experiment, by which we all can easily
place ourselves in a condition somewhat like that of Harris,
where all or nearly all sensation of colour had vanished.
If some carbonate of soda be ignited in the flame of a Bun-
sen burner, it will furnish an abundance of pure homoge-
neous light of an orange-yellow hue. This light is quite
bright enough to illuminate objects in a darkened room,
but all distinctions of colours vanish, light and shade only
remaining. A red rose exhibits no more colour than its
leaves ; gayly painted strips of paper show only as black or
white or grey ; their colours can not even be guessed at.
The human face divested of its natural colour assumes an
appearance which is repulsive, and the eye in the absence
of colour dwells on slight defects in the clearness and
smoothness of the complexion. If now an ordinary gas-
burner be placed near the soda flame, and allowed at first
.to burn with only a small flame, objects will resume their
natural tints to some slight extent, and begin again faintly
to clothe themselves with pleasant hues, which will deepen
as more light is furnished, till they finally seem fairly to

* The amount of time necessary for Tision. " American Journal of Sci-
ence," September, 1871.

APPENDIX TO CHAPTER VIII.

103

glow with radiant beauty. Those who have never wit-
nessed an experiment of this kind have but little conception
how great would be to us the loss of our sense for colour,
or how dreary the world would seem, divested of the fasci-
nating charm which it casts over all things.

APPENDIX TO CHAPTER VIII.

Maxwell has published an account of his rather elaborate
examination of the case of one of his students, who was colour-
blind to red.* ■ An apparatus was employed by which the pure
colours of the spectrum could be mixed in any proportion ; these
colours were then mingled by the colour-blind person, in such a
proportion as to produce to his eyes the effect of white. In this
way the following equation was obtained: 33-7 green + 33-1 blue
= white. Maxwell then, employing the same colours, obtained his
own or a normal equation, which was ; 26 green + ST'-i blue + 22'6
red = white. If we combine these equations by subtraotipn, we
obtain : 32'6 red — 7'7 green + 4'3 blue = D ; D being the missing
colour not perceived by the colour-blind. The sensation, then.

/6R^

/'blX

ABCDEF G H

Fig. so.— Curves of a Colour-blind Person. (Maxwell.)

which Maxwell had in addition to those of the colour-blind person
was somewhat like that of a full red, but different from it in that
the full red was mixed with 7'7 green, which had to be removed
from it, and 4'3 of blue substituted. The missing colour, then, ac-
cording to this experiment, was a crimson-red. Even normal eyes
vary a little, and, if this examination had been made by Maxwell's

* " Philosophical Transactions " for 1860, vol. cl., p. 78.

104

MODERN CHKOMATICS.

assistant (observer K), the result would have been a red mixed with
less blue, consequently a colour much more like the red of the
spectrum. From experiments of this kind Maxwell was able to
construct the curves of intensity of the two fundamental colours
which are perceived by those who are colour-blind to red ; these
curves are shown in Fig. 30. The letters A, B, 0, D, etc., mark
the positions of the fixed lines in the solar spectrum ; the curved
line marked GE exhibits the intensity of the green element, the
line marked BL that of the blue or violet. It wUl be noticed that
the green sensation attains its maximum about half way between
the lines D and E, that is, in the yellowish green; whUe the high-
est point of the other curve is about half way between F and G,
that is, in the blue space. Maxwell also constructed similar inten-
&ity curves for a normal eye; they are represented in Fig. 31, the

BED

\ Gff.

BL.

Fig. 81. — Curves of Normal Eye. (Maxwell.)

curve for red being indicated by a heavy Une, the others as above.
The green and blue curves have about the same disposition as with
the colour-blind person, whUe the red attains its maximum between
0 and D, but nearer D — ^that is, in the red-orange space.

A set of observations was also made by Maxwell on the same
colour-blind gentleman, with the aid of coloured disks in rapid ro-
tation ; and, from the colour equations thus obtained, the positions
of the colours perceived by him were laid down in Newton's dia-
gi-aro, in a manner similar to that explained in the appendix to
Chapter XIV. In Fig. 32, V shows the position assumed for red
or vermilion ; U, that of ultramarine- blue ; and G,. that of emerald-

APPENDIX TO CHAPTER VIII. 105

green. They are placed according to Maxwell's method, at the
three angles of an equilateral triangle. W would be the position of
white for a normal eye, and Y that of chrome-yellow. D is the
position of the defective colour, which Maxwell was able to imitate
by mingling, by the method of rotating disks, 86 parts of vermilion
and 14 of ultramarine-blue. A line drawn from D through W con-
tains along its length the various shades of grey and the white of
the colour-blind. The grey which they perceive when green and

Fia.82.— Newton'B Dlwram for a Person Fio. 88.— Newton's Diagram for a Per-

Colour-blind to Red. (Maxwell.) son Oolour-bllnd to the Fundamental

Bed.

blue are mixed lies at w; the white of white paper, i. e., a more
luminous grey, was on the same line but considerably farther along
outside.

It may pei'haps be as well to add to the above one or two re-
marks concerning the construction of Newton's diagram for the
colour-blind. Let us suppose that the pure colours of the spectrum
are employed, and that the missing colour is the fundamental red ;
we then place the fundamental green at G, Fig. 33, the fundamental
blue or violet at U, and the missing red at D. Then along the line
U Q- will lie mixtures of blue and green, and at w will be the white
of the colour-blind person. Along the line D G will be situated
various shades of green, from dark green to bright green, the latter
colour predominating as we approaolj G. Along the line D U we
shall have various shades of blue, from bright blue to dark blue,
the colour being very dark near D and very bright near U. A line
like the dotted one (Fig. 33) will contain various shades of green,
fi'om light green to dark green, but none of them so intense as

106 MODERN CHROMATICS.

those situated along the line D G- ; in other words, they all will be
mingled with what the colour-blind call white.

If the defective colour-sensation is supposed still red, but to be
only partially absent, the diagram takes the form indicated in Fig. 34: ;
that is, red, instead of occupying the position at one of the angles
of an equilateral triangle, wiU be moved up toward the centre to E'.
White will also be shifted from W to w, and the white of a person
thus affected would appear to the normal eye of a somewhat green-
ish-blue hue. Between D and K' lie, so to speak, mixtures of red
with darkness, and along the line E' G- wiU be various mixtures
of red and green, in which, according to a normal eye, the green
element quite predominates ; that is to say, their orange is more

Fig. 84. — Newton^B Diagram for a Per- Fig. 35. — ^Newton's Diagram for Lamp-

fion partially Colour-blind to Bed. light Illuxninatlon.

like our yellow, their yellow like our greenish yellow, etc. Along
the line E' U will be their mixtures of red and blue, or a series of
purples, which will be more bluish than ours.

The condition of the normal eye by lamp-light is shown in Fig.
35. The blue or violet is moved from U, its position by daylight,
up to M ; white is moved from W to w — that is, into a region that
would be called by daylight yellow. Yellow itself, T, is not far
Irom this new representative of white, and consequently by candle-
light appeal's always whitish. In the purples, along the line E u,
the red element predominates ; and in the mixtures of green and
blue, along the line G u, the green constituent has the upper hand.

If we were colour-blind to every kind of light except red, then
the colour diagram would assume a form similar to that shown in
Fig. 86, D representing the darkest red perceptible to eyes so consti-
tuted. This sensation would be brought about by pure feeble red

APPENDIX TO CHAPTER VIH.

107

light, or by a mixture of intense green and blue light, or by either
of the latter. As we advance from D toward E, the red light
gains in brightness, and out at v> becomes very bright and stands
for white. When a red glass is held before the eyes, something
approximating to this kind of vision is produced.

Fio. 86,— Newton's Diagram for Persons Colonr-bltod to Green and Violet.

CHAPTER IX.

THE COLOUR THEORY OF YOUNG AND HELMHOLTZ.

It is well known to painters tliat approximate represen-
tations of all colours can be produced by the use of very-
few pigments. Three pigments or coloured powders wUl
suifiee, a red, yellow, and a blue ; for example, crimson-
lake, gamboge, and Prussian blue. The red and yeUow
mingled in various proportions will furnish different shades
o^ orange and orange-yeUow ; the blue and yellow will
give a great variety of greens ; the red and blue aU the
purple and violet hues. There have been instances of
painters in water-colours who used only these three pig-
ments, adding lampblack for the purpose of darkening them
and obtaining the browns and greys. Now, though it is
not possible in this way to obtain as brilliant representatives
of the hues of nature as with a less economical palette, yet
substitutes of a more or less satisfactory character can actu-
ally be produced in this manner. These facts have been
known to painters from the earliest ages, and furnished the
foundation for the so-called theory of three primary colours,
red, yellow, and blue. The most distinguished defender in
modern times of this theory was Sii- David Brewster, so
justly celebrated for his many and brilliant optical discov-
eries. He maintained that there were three original or fun-
damental kinds of light, red, yellow, and blue, and that by
their mixture in various proportions all other kinds of col-
oured light were produced, in the manner just described for
pigments. Brewster in fact thought he had demonstrated

THE COLOUK THEORY OF YOUNG AND HELMHOLTZ. 109

the existence in the spectrum itself of these three sets of
fundamental rays, as well as the absence of all others ; and
his great reputation induced most physicists for more than
twenty years to adopt this view, Airy, Melloni, and Draper
alone dissenting. This theory of the existence of three
fundamental kinds of light, red, yellow, and blue, is found
in all except the most recent text books on physics, and is
almost universally believed by artists.- Nevertheless, it
will not be diiEcult to show that it is quite without founda-
tion. If we look at the matter from a theoretical point of
view, we reach at once the conclusion that it can not be
true, because outside of ourselves there is no such thing as
colour, which is a mere sensation that varies with the
length of the wave producing it. Outside of and apart from
ourselves, light consists only of waves, long and short — in
fact, of mere mechanical movements ; so that Brewster's
theory would imply that there were in the spectrum only
three sets of waves having three different lengths, which
we know is not the case. If we take up the matter experi-
mentally, we meet with no better result. According to the
theory now under consideration, green light is produced by
mixing blue and yellow light. This point can be tested

Fig. 87.~MaxweIl'8 Disks. Blue and Yellow Disks in the Act of being combined.

with Maxwell's coloured disks. A circular disk, painted
with chrome-yellow and provided with a radial slit, is to be
combined with one prepared in the same way and painted
with ultramarine-blue. Fig. 37 shows the separate disks,
and in Pig. 38 they are seen in combination. If the com-
pound disk be now set in quite rapid rotation, the two kinds

110

MODERN CHKOMATICS.

of coloured light will be mmgled, and the resultant tint can
be studied. It will not be green, but yellowish grey or

Fig. 88— Blue and Yellow Disks in Combination.

reddish grey, according to the proportions of the two col-
ours. These disks of Maxwell are ingeniously contrived so
as to allow the experimenter to mingle the two colours in

Fio. 89.— Apparatus of Lambert for mixing Coloured Light.

any desired proportion ; but, vary the proportions as we
may, it is impossible to obtain a resultant green hue, or in-

THE COLOUR THEOKY OF YOUNG AND HELMHOLTZ. Ill

deed anything approaching it. Another way of making
this experiment is simply to use a fragment of window-glass
of good quality, as was done by Lambert and Helmholtz.
This apparatus is shown in Fig. 39. The glass is supported
in a vertical position about ten inches above a board painted
black, and on either side of it are placed the coloured
papers. The blue paper is seen directly through the glass,
while the light from the yellow paper is first reflected from
the glass and then reaches the eye. The result is that the
two images are seen superimposed, as is indicated in Fig.
40. The relative luminosity or brightness of the two im-

Fio. 40.— Eesutt furnished by the Apparatus.

ages can be varied at will ; for instance, moving the papers
further apart causes the blue to predominate, and bringing
them nearer together produces the reverse effect. In this
manner the resultant tint may be made to run through a
variety of changes, which will entirely correspond to those
obtained with the two circular disks ; but, as before, no
tendency to green is observed. Helmholtz has pushed this
matter still further, and has studied the resultant hues pro-
duced by combining together the pure colours of the spec-
trum. The following experiment, which is easy to make,
will give an idea of the mode of proceeding : A blackened
screen of pasteboard is pierced with two narrow slits, ar-
ranged like those in Fig. 41. The light from a window is
allowed to shine through the two slits and to fall on a prism
of glass placed just before the eye, and distant from the
slits about a metre. Then, as would be expected, each slit

112

MODEEN CHROMATICS.

furnishes a prismatic spectrum, and owing to the disposition
of the slits, the two spectra will overlap as shown in Fig.
"42, which represents the red space of one spectrum falling

Fig. 41.— Two Slits arranged for mixing Two Spectra.

on the green space of its companion. By moving the slits fur-
ther apart or nearer together, all the different kinds of light
which the spectrum contains may thus be mingled. Using
a more refined apparatus, Helmholtz proved that the union
of the pure blue with the pure yellow light of the spectrum
produced in the eye the sensation, not of green, but of
white light. Other highly interesting results were also
obtained by him- during this investigation ; these will be

RED

0. >

GREEN
ORMQE

BLUE

^^^^^1

is.fr.

B.miET

■

RED

K; 6REEN

BLUE VIOLET

Wm. 42.— Two Overlapping Spectra. Bed and Green are mixed, also Violet and Blue, etc.

considered in the following chapter, but in the mean while
it is evident that this total failure of blue and yellow light
to produce by their mixture green light is necessarily fatal

THE COI/OUR THEOEY OF YOUNG AND HELMHOLTZ. 113

to the hypothesis of Brewster. Helmholtz also studied the
nature of the appearances which misled the great English
optician, and showed that they were due to the fact that he
had employed an impure spectrum, or one not entirely free
from stray white light.

As has been remarked above, there can be in an objec-
tive sense no such thing as three fundamental colours, or
three primary kinds of coloured light. In a totally differ-
ent sense, however, something of this kind is not only pos-
sible, but, as the recent advances of science show, highly
probable. We have already seen in a previous chapter that
in the sp^lar spectrum the eye can distinguish no less than
a thousand different tints. Every small, "minute, almost
invisible portion of the retina of the eye possesses this
power, which leads us to ask whether each atom of the ret-
ina is supplied with an immense number of nerve fibrils for
the reception and conveyance of this vast number of sensa-
tions. The celebrated Thomas Young adopted another
view : according to him, each minute elementary portion of
the retina is capable of receiving and transmitting three
different sensations ; or we may say that each elementary
portion of its surface is supplied with three nerve fibrils,
adapted for the reception of three sensations. One set of
these nerves is strongly acted on by long waves of light,
and produces the sensation we call red ; another set re-
sponds most powerfully to waves of medium length, produc-
ing the sensation which we call green ; and finally, the third
set is strongly stimulated by short waves, and generates
the sensation known as violet. The red of the spectrum,
then, acts powerfully on the first set of these nerves ; but,
according to Young's theory, it also acts on the two other
sets, but with less energy. The same is true of the green
and violet rays of the spectrum : they each act on all three
sets of nerves, but most powerfully on those especially
designed for their reception. All this will be better un-
derstood by the aid of the accompanying diagram, which is

114

MODERN CHROMATICS.

taken from Helmholtz's great work on " Physiological Op-
tics." In* Fig. 43, along the horizontal lines 1, 2, 3 are

Fig. 43. — Curves showing the Action of the Different Colours of the Spectrum on the
Three Sets of Nerve Fibrils. (Helmholtz.)

placed the colours of the spectrum properly arranged, and
the curves above them indicate the degree to which the
three kinds of nerves are acted on by these colours. Thus
we see that nerves of the first kind are p'owerf ully stimu-
lated by red light, are much less affected by yellow, still
less by green, and very little by violet light. Nerves of
the second kind are much affected by green light, less by
yellow and blue, and still less by red and violet. The third
kind of nerves answer readily to violet light, and are suc-
cessively less affected by other kinds of light in the follow-
ing order : blue, green, yellow, orange, red. The next
point in the theory is that, if all three sets of nerves are
simultaneously stimulated to about the same degree, the
sensation which we calt white will be produced. These are
the main points of Young's theory, which was published as
long ago as 1802, and more fully in 1807. Attention has
within the last few years been called to it by Helmholtz,
and it is mainly owing to his labours and those of Maxwell
that it now commands such respectful attention. Before
making an examination of the evidence on which it rests,

THE COLOUR THEORY OP YOUNG AND HELMHOLTZ. 115

and of its applications, it may be well to remember, as
Helmholtz remarks, tbat the choice of these three particular
colours, red, green, and violet, is somewhat arbitrary, and
that any three could be chosen which when mixed together
would furnish white light. If, however, the end and mid-
dle colours of the spectrum (red, violet, and green) are not
selected, then one of the three must have two maxima, one
in the red and the other in the violet ; which is a more
complicated, but not an impossible supposition. The only
known method of deciding this point is by the investigation
of those persons who are colour-blind. In the last chapter
it was shown that the most common kind of this affection
is colour-blindness to red, which indicates this colour as
being one of the three fundamental sensations. But, if we
adopt red as one of our three fundamental colours, of
necessity the other two must be green and violet or blue-
violet. Red, yellow, and blue, for example, will not pro-
duce white light when mingled together, nor will they
under any circumstances furnish a green. Red, orange,
and blue or violet would answer no better for a fundamental
triad. In the preceding chapter it was also shown that
colour-blindness to green exists to some extent, though by
no means so commonly as the other case. Hence, thus far,
the study of colour-blindness has furnished evidence in
favour of the choice of Young, and its phenomena seem
explicable by it.

Let us now examine the explanation which the theory
of Young furnishes of the production of the following
colour- sensations, which are not fundamental, viz. :

Orange-red.
Eed-ovange.
Orange-yellow.

Yellow.

Greenish-yellow.

Yellowish-green.

Bluish-green.

Cyan-blue.*

Ultramarine-blue.

Starting with yellow, we find that,. according to the theory
under consideration, it should be produced by the joipt

* Cyan-blue is a greenish-blue.

116 MODERN CHKOMATICS.

stimulation of the red and green nerves ; consequently, if
we present simultaneously to the eye red and green light,
the sensation produced ought to be what we call yellow.
This can be most perfectly accomplished by mixing the red
and green light of the spectrum ; it is possible in this way
to produce a fair yellow tint. The method of rotating
disks furnishes, when emerald-green and vermilion are em-
ployed, a yellow which appears rather dull for two reasons :
first, because the pigments which we call yellow, such as
chrome-yellow or gamboge, are, as wUl hereafter be shown,
relatively more brilliant and luminous than any of the -red,
green, blue, or violet pigments in use ; so that these bright-
yellow pigments stand in a separate class by themselves.
This circumstance influences our judgment, and, finding the
resultant yellow far less brilliant than our (false) standard,
chrome-yellow, we are disappointed. The second reason is,
that green light stimulates, as before mentioned, the violet
as well as the green nerves ; hence all three sets of nerves
are set in action to a noticeable extent, and the sensation of
yellow is mingled with that of white, and consequently is
less intense than it otherwise would be. When the green
and red of the spectrum are mingled, we have at least not
to contend with a false standard, and only the second reason
comes into play, and causes the yellow thus produced to
look as though mingled with a certain quantity of white.
It was found by the lamented J. J. Mtiller that green light
when mingled with any other coloured light of the spec-
trum diminished its saturation, and caused it to look as
though at the same time some white light had been added.
This is what our fundamental diagram (Fig. 43) would lead
us to expect ; it is quite in consonance with the theory of
Young and Helmholtz.

Having now accounted for the fact that the yellow pro-
duced by mixing red and green light is not particularly
brilliant, it will be easy to show how several of the other
colour-sensations are generated. If, for instance, we dhnin-

THE COLOtTR THEORY OF YOTTNG AND HELMHOLTZ. 117

ish the intensity of the green light in the experiment above
mentioned, the resultant hue will change from yellow to
orange, red-orange, orange-red, and iSnally to pure red.
These changes are best followed by using the coloured
light of the spectrum, but may also be traced by the help
of Maxwell's disks (Fig. 38), or by the aid of the glass
plate of Helmholtz (Fig. 39). On the other hand, if, in
the experiment now under consideration, the green light be
made to preponderate, the resultant yellow hue will pass
into greenish yellow, yellowish green, and finally green.
This accounts for the production of more than half the col-
our-sensations in the list above given, and the remaining
ones, such as ultramarine, cyan-blue, and bluish green, can
be produced in the same way by mingling in proper pro-
portions green and violet light, using any of the methods
above mentioned.

In the cases thus far considered we have presented to
the eye mixtures of two different kinds of coloured light,
or, to speak more accurately, two kinds of light differing in
wave-length ; it now remains for us to account for the pro-
duction of colour-sensations in those cases where the eye is
acted on only by one kind of coloured light, or by light
having one wave-length. In the case of red, green, or vio-
let light, the explanation of course lies on the surface : the
red light stimulates powerfully the red nerves and produces
the sensation we call red, and so of the others. But this
does not quite exhaust the matter ; for, according to the
theory of Young and Helmholtz, this same red light also
acts to some extent on the green and violet nerves, and si-
multaneously produces to some small degree the sensations
we call green and violet. The result then, according to the
theory, ought to be the production of a strong red sensa-
tion, mingled with much weaker green and violet sensa-
tions ; or, in other words, even when the eye is acted on by
the pure red light of the spectrum, this red light ought to
appear as though mingled with a little white light, even if

118 MODERN CHROMATICS.

none is actually present. Experiment confirms this theo-
retical conclusion, and here again decides in favour of the
correctness of our theory. The simplest way of making
the experiment would be to temporarily remove, were it
possible, the green and violet nerves froin a portion of the
retina of the eye, and then to throw on the whole retina the
pure red light of the spectrum. This red light ought then
to appear more intense and saturated when falling on the
spot from which the green and violet nerves had been re-
moved than when received on the rest of the retina, where
all three kinds of nerves were present. Now, though we
can not actually remove the green and violet nerves from a
spot in the retina, yet we can by suitable means tire them
out, or temporarily exhaust them, so that they become to
a considerable extent insensitive. If a small spot of the
retina be exposed for a few moments to a mixture of green
and violet light so combined as to appear bluish green, the
green and violet nerves will actually become to a consider-
able extent inoperative ; and, when the eye is suddenly
turned to the red of the spectrum, this spot of the retina
will, if we may use the expression, experience a more
powerful and purer sensation of red than the surrounding
unfatigued portions, where the red will look as if diluted
with a certain amount of white light. From this experi-
ment of Helmholtz it appears, then, that it is actually pos-
sible to produce by artificial means colour-sensations which
are more powerful than those ordinarily generated by the
light of the spectrum — a point to which we shall return in
the following chapter.

Having accounted now for the production of the colour-
sensations red, green, and violet by red, green, and violet
light, and noticed an interesting peculiarity connected with
this matter, we pass on to the others. Taking up the yel-
low of the spectrum, we find that it can be produced by the
action on the eye of waves of light intermediate in length
between those which give the sensations red and green.

THE COLOUR THEORY OF YOUNG AND HELMHOLTZ. ng

These waves are too short to act very powerfully on the
red nerves, and too long to set into maximum activity the
green nerves, but they set both into moderate action ; the
result of this joint action of the two sets is a new sensation,
which we call yellow. Furthermore, it may be remarked
that the waves of the light called yellow are far too long to
produce any but a feeble effect on the violet nerves ; they
affect them less than green light does. Prom this it results
that the sensation of yellow, when directly produced by
the yellow light of the spectrum, is less mingled with that
of white, and is purer than is the case when it is brought
about by mixing red light with green in the manner before
described. And this explanation may serve to account for
the fact that it is impossible, by mixtures of red and green
light taken from the spectrum, to produce a yellow light as
pure and brilliant as the yellow of the spectrum. Let iis
suppose, in the next place, that, instead of presenting to
the eye the yellow of the spectrum, we act on it by the
light belonging to one of the other spaces — the blue, for
example. The explanation is almost identical with that
just given for the yellow : the waves constituting blue light
being too short to powerfully affect the green nerves, and
too long to accomplish this with the violet nerves, both
green and violet nerves are moderately affected, giving the
sensation we call blue. Meanwhile the blue light produces
very little action on the red nerves, and hence very little of
the sensation of white is mingled with that of blue ; and
consequently this blue hue is more saturated than when
produced by the actual mixture of green and violet light.
In fact, J. J. Mtlller found that green light, when mingled
with light from any other part of the spectrum, produced a
hue which was less saturated and more whitish than the
corresponding tint in the spectrum which the mixture imi-
tated. The production of all the other colour-sensations
obtained by looking at the spectrum is explained in the
same way by our theory. From all this one interesting
6

120 MODEEN CHROMATICS.

conclusion can he drawn, viz. : that there are two distinct
ways of producing the same colour-sensation ; for we have
seen that it may be accomplished either by presenting to
the eye a mixture of green and yiolet light, or simply one
kind of light, the waves of which are intermediate in length
between those of green and violet. The eye is quite unable
to detect this difference of origin, although a prism reveals
it instantly.

Having examined thus, with a degree of detail which
may have seemed tedious, the mode in which colour-sensa-
tions are accounted for by the theory of Young and Helm-
holtz, we pass to another point. In order to give more
exactness to this theory, it is necessary to define with some
degree of accuracy the three fundamental colours ; for there
is a great variety of reds, greens, and violets. Helmholtz,
as the result of his first investigation, selected a red not far
from the end of the spectrum, a full green and violet ; in
other words, the tints chosen were the middle and end col-
ours of the spectrum. Maxwell, who made a series of beau-
tiful researches on points connected with Young's theory,
was led to adopt as the fundamental colours a red which in
the spectrum lies between the fixed lines C and D, and is
distant from C just one third of the distance between C
and D. This is a scarlet-red with a tint of orange, and is
represented by some varieties of vermilion. His green is
situated between E and F, being distant from E by one
quarter of the distance between E and P. This colour
finds among pigments an approximate representative in
emerald-green. Instead of adopting a full violet, Maxwell
selected a violet-blue midway between the lines F and G,
which is represented tolerably by artificial ultramarine-blue.
By subjecting the results of experiments on the spectrum
to calculation, it is possible to fix on the position of one of
the fundamental colours, viz., the green. Thus Charles S.
Pierce, using data given in Maxwell's paper, obtained for
this colour a slightly different result from that just men-

THE COLOUR THEORY OF YOUNG AND HELMHOLTZ. 121

tioned.* According to his calculations, the fundamental
green has a wave-length of 524 ten-millionths of a milli-
metre, and is situated between the lines E and 5, being one
third of the distance E b from E, whereas Maxwell's green
is just beyond b. J. J. Mtlller, who conducted an impor-
tant investigation on this subject by a quite different meth-
od, arrived at a somewhat different result for the position
of the green, and assigned to it a wave-length of 506-3 ten-
millionths of a millimetre. This position in the spectrum
is nearer the blue than the positions given by Maxwell and
Pierce, and the tint is more of a bluish green. Again, von
Bezold, basing his calculations on the experimental results
of Helmholtz and J. J. MflUer, reached a conclusion not
differing much from those of Maxwell or Pierce. He se-
lects a green in the middle of the normal spectrum between
E and b, but nearer b. /None of these results differ very
greatly ; in fact, the differences can hardly be well indicated
in a spectrum of the size of this page. All these greens
may be imitated by using the pigment known as emerald-
green, alone or mixed either with a small quantity of
chrome-yellow or cobalt-blue. Hence all these green hues
are of the most powerful or, as artists would say, over-
powering character.

The exact determination of the other two fundamental
colours is a more difficult matter, so that even the advocates
•of Young's theory have not entirely agreed among them-
selves upon the exact colours, Maxwell taking ultramarine-
blue, Helmholtz and J. J. Mtlller violet, as the third funda-
mental. These fundamental colours are among the most
saturated and intense of those furnished by the spectrum.
Compared with them, the blue of the spectrum is a feeble
tint, so that it has often been remarked by Rutherfurd that,
in comparison with the other colours, it appears of a slaty
hue. The greenish yellow is also feeble ; and, as is well

* "Proceedings of the American Academy of Arts and Sciences, 1873."

122 MODERN CHROMATICS.

known, pure yellow is found in tlie spectrum in very small
quantity and of no great intensity. The orange-yellow is
also much weaker than the red, and the orange only be-
comes strong as it approaches redness in hue. From this it
very naturally follows that, if a normal spectrum is cast on
a white wall in a room not carefully darkened, scarcely
more than the three fundamental colours will be discerned,
red, greefi, and blue-violet ; the other tints can with some
difficulty be made out, but at first sight they strike the un-
prejudiced observel- simply as the places where the three
principal colours blend together. The representatives of
the fundamentals among pigments are also those which
surpass all others in strength and saturation. One of the
fundamental colours, red, is used without much difficulty in
painting and decoration ; the other two are more difficult
to manage, particularly the green. The last colour, even
when subdued, is troublesome to handle in painting, and
many artists avoid 'it as far as possible, or admit it into
their work only in the form of low olive-greens of various
shades. When the tint approaches the fundamental green,
and is at the same time bright, it becomes at once harsh
and brilliant, and the eye is instantly arrested by it in a
disagreeable manner.

NOTE TO CHAPTER IX.

YouifG does not appear to be the first who proposed red, green,
and violet as the three fundamental colours. As far back as 1792
"Wilnsch was led to the same result by his experiments on mixtures
of the coloured rays of the spectrum. The title of his -work is
"Versnohe und Beobachtungen tlber die Farben des Lichtes"
(Leipsic, 1792). An abstract of the contents is contained in the
" Annales de Chimie," LXIV., 135.

A. M. Mayer has recently called attention to the way in which
Young was led to adopt red, green, and violet as the three funda-
mental colours, and has shown that Young at first " selected red,

NOTE TO CHAPTER IX.

123

yellow, and blue as the three simple colour-sensations ; second, that
he subsequently modified his hypothesis, and adopted red, green,
and violet as the three elementary colour-sensations, showing that
up to the date of this change of opinion all of his ideas on the sub-
ject were hypothetical, and not based on any observations of his
own or others ; third, that this change of opinion as to the three
elementary colours was made on the basis of a misconception by
WoUaston of the nature of his celebrated observation of the dark
lines in the solar spectrum, and also on the basis of an erroneous
observation made by Young in repeating WoUaston's experiment ;
fourth, that Young subsequently tested his hypothesis of colour-
sensation, and found that it was in accord with facts reached by
experiment, and that these experiments then vindicated his hy-
pothesis and raised it to the dignity of a theory." ("American
Journal of Science and Arts," April, 1875.)

Fig. 43 (page 114) shows the intensities of the three primary sensa-
tions, red, green, and violet, as estimated by Helmholtz. The intensi-
ties were afterward measured by Maxwell, and found to differ slightly
in the case of diflFerent eyes. In Fig. 44 the letters G D E F G de-
note the fixed lines of the solar spectrum ; the curve ERE, the

Fio. 44.— Curves showing the Intensity of the Fundamental Sensations in Different
Parts of the Solar Spectrum. (Maxwell.)

intensity of the sensation of red in different parts of the spectrum ;
G G G is the curve for the green, and B B B that for the violet-blue
sensation. Fig. 81 (page 104) shows the same curves as obtained by
another observer. (" Philosophical Transactions " for 1860, vol. cl.)

CHAPTER X.

ON THE MIXTURE OF COLOURS.

Those who watch a painter at work are astonished at
the vast number and variety of tints which can be made by
mixing in varying proportions a very few pigments : from
red and yellow there is produced a great series of orange
tints ; yellow and blue furnish a multitude of green hues ;
blue and red, a series of purples. Thp results seem almost
magical, and we justly admire the skill and knowledge
which enable him in a few seconds accurately to match any
colour which is within the compass of his palette. As we
continue our observations we soon find that the matter is
more complicated than it appeared at first sight, each pig-
ment having a particular set of properties which it carries
into its mixtures, and these properties being by no means
fully indicated by its mere colour. Thus, some blue pig-
ments furnish fine sets of greens, while others, as beautiful
and intense, yield only dull olive-greeris ; some reds give
glowing purples, whUe from others not less bright it is pos-
sible to obtain only dull, slaty purples. Before touching
on these complicated- cases it will be well to study this sub-
ject under its simplest aspects, and to content ourselves for
the present with making an examination of the effects
which are produced by mixing light of different colours.
This can not be brought about by mixing pigments, as was
for a long time supposed. In some cases the mixture of
pigments gives results more or less like those produced by

ON THE MIXTURE OF COLOURS.

125

the mixture of coloured light, but as a general thing they
differ, and in some cases the difference is enormous. In
the previous chapter, for instance, it was shown that while
the mixture of yellow with blue pigments produced inva-
riably a green hue of varying intensity, the mixture of blue
with yellow light gave a more or less pure white, but under
no circumstances anything approaching green. The mix-
ture of two masses of coloured light can easily be effected
in a simple manner so as to be exhibited readily to a large
audience. Two magic lanterns are to be employed, as
shown in Fig. 45, the usual slides being replaced by plates

Fig. 46. — Two Magic Lanterns castiug Yellow and Blue Light on the same Screen,
where it forms White.

of coloured glass, as indicated. Each lantern then will fur-
nish a large bright circle of coloured light, which can be
projected on a white screen, the room in which the experi-
ments are made being first darkened. In this way it will
be found that violet-blue light when mixed with green
light gives blue or greenish-blue light, according to the
proportions of the two constituents ; green ■with red gives
various hues of orange or whitish yellow, instead of a set
of dull, indescribable shades of greenish, reddish, or brown-
ish grey, . as is the case with pigments. These and many

126 MODERN CHROMATICS.

Other beautiful experiments on the mixture of coloured
light can easily be made ; and even the effects produced by
varying the intensity of either of the masses of coloured
light can readily be studied by gradually diminishing the
brightness of one of the coloured circles, the other remain-
ing constant.

To all these experiments it may be objected that we are
not using coloured light of sufficient purity ; that our yel-
low glass transmits, as was shown in Chapter VII., not only
yellow but red, orange, and green light ; and that the other
stained glasses are not much better off in this respect.
Hence, to meet all such objections, physicists have been
finally obliged to conduct their researches in this direction
on the pure coloured rays of the spectrum. The difficulties
encountered in the use of this method are much greater,
but the results so obtained are far more precious. Very
beautiful investigations of this character have been made
by Helmholtz, Maxwell, and J. J. Muller. The general
character of their results is something like this : By mixing
two kinds of pure coloured light they obtained, as a general
thing, light having a colour different from either of the
original ingredients ; for example, red and yellowish green
gave an orange hue which looked in all respects like the
pure orange of the spectrum ; also, in this new orange it
was impossible by the eye to detect the presence of either
red or yellowish-green light. This was true of all mixtures ;
in no case could the original ingredients be detected by the
eye. In this respect the eye differs from the ear ; for by
practice it is possible with the unaided ear to analyze up
compound sounds into the elements which compose them,
at least to some extent. It was also ascertained that the
same colour could be produced in several different ways —
that is, by combining together different pairs of spectral
colours. Thus, violet with cyan-blue gave an ultramarine
hue, but violet gave the same colour when mixed with
bluish-green, or even with green ; in the last case the tint

ON THE MIXTURE OF COLOURS.

127

.Eed.

> Eed-oran/je.

"Orange.

" Orange-yeUow.

-Yellow.

Greenish-yellow

and
TeUo^vlsh.green.

^ Green and
' Blue-green.

k Cyan-blue.

was somewhat whitish. By mixing certain colours of the
spectrum, it was found that one
new colour or colour-sensation
could be produced which origi-
nally was not furnished by the ^
plain pure spectrum itself ; we re- c
fer to purple, or rather to the
whole class of purples, ranging
from violet-purple to red-purple, j.
These are produced by mixing the
end colours of the spectrum, red
and violet, in varying proportions.
Furthermore, mixtures of certain
colours of the spectrum gave rise ^
to white ; this was true, for ex- *
ample, of fed and bluish-green,
and of yellow and ultramarine-
blue. The white in these two f
cases, though so different in origin,
had exactly the same appearance
to the eye. Again, by mixing
three or more spectral colours no
new hues were produced, but sim-
ply varieties of those which could
be obtained from two colours.

Such is the general character g
of the results obtained by mixing
together masses of pure coloured
light ; we propose now to enter a
little more into detail respecting
this very interesting matter, and
to examine the laws which control
the production of the resultant
hues. ^

It was ascertained by Mtlller,
who worked under the direction

V Blue and
/ Blue-violet,

> Violet.

Fig. 46.— Prismatic Speotrum.

138 MODERN CHROMATICS.

of Helmholtz, that all the colours of the spectrum, Fig. 46,
from red to yellowish-green, gave by mixture resultant
hues which were always identical with some of the colours
situated between red and yellowish-green, thus :

Table I.

Red and yellowish-green gave Orange or yellow.*

Red and yellow gave Orange.

Orange and yellowish-green gave Yellow.

The effect of the mixture in these cases was to produce
colours which were, to all appearance, as pure as the cor-
responding colours of the spectrum itself.

Furthermore, all colours of the spectrum from violet to
bluish-green furnished mixtures corresponding to the col-
ours contained between these limits, thus :

Table II.

Bluish-green and ultramarine-blue gave. . . Cyan-blue.

Bluish-green and violet gave Cyan-blue or ultramarine-blue.*

Violet and cyan-blue gave Ultramarine-blue.

In these cases also the resulting tints could not be distin-
guished from the corresponding spectral colours. The re-
sults thus far are simple in character, and easily remem-
bered by any one who recollects the arrangement of the
colours of the spectrum.

On the other hand, green, when mixed with any colour
of the spectrum, gave a resultant colour, which was less
saturated or intense, and appeared more whitish, than the
corresponding spectral tint, thus :

Table III.

f Orange, \

Green and red gave } Yellow, C whitish.

' Yellowish-green, )

* According to the proportions.

ON THE MIXTURE OF COLOURS. 129

Green and yellow gave Yellowish-green — whitish.

Green and cyan-blue gave Bluish-green — whitish.

( Ultramarine-blue, J
Green and violet gave •{ Cyan-blue, > whitish.

' Bluish-green, )
Yellowish-green and bluish-green gave. Green — very whitish.

Mtlller made a careful determination of the position in the
spectrum of the green which had the greatest effect in
diminishing the saturation, and consequently was most in-
fluential in generating pale or whitish tints. It was situ-
ated between the fixed lines b and F, at one third the dis-
tance between b and F, measured from b. This colour is a
bluish-green, and can be imitated by mixing emerald-green
with a small quantity of cobalt-blue. According to Mtlller,
as already stated, this is the fundamental green : its wave-
length is 506 '3 ten-millionths of a millimetre.

Having considered the effects produced by mixing the
colours of the spectrum situated on either side of the green,
and also the effects produced by green itself in mixture, it
remains to examine the mixtures of the colours located
right and left of green, thus :

Table IV.

Bed and ultramarine-blue gave. . . Violet — slightly whitish.

Red and cyan-blue gave Ultramarine or violet — whitish.

Orange and violet gave Red — whitish.

Red and violet gave Purple — whitish.

Orange and ultramarine gave. . . . Purple — whitish.

These results may at first sight not seem as simple and ob-
vious as those mentioned above, but, when the arrangement
of the colour-diagram* has been explained, it will be seen
that they are strictly analogous to the cases before given.

It may have been noticed by the reader that the series
of pairs given in the tables thus far do not entirely exhaust

* See Chapter XIV.

130 MODERN CHROMATICS.

all possible combinations of the spectral colours. In tbe
cases which remain, however, the effect of mixture is not
the production of coloured but of white light, thus :

Table V.

Red and bluish-green* gave White.

Orange and cyan-blue gave White.

Yellow and ultramarine gave White.

Greenish-yellow and violet gave White.

Hence these are called complementary colours, and, on ac-
count of their importance, a separate chapter will be devoted
to them. Green finds no simple complementary colour in
the spectrum ; it requires a mixture of red and violet, or
the colour called purple.

These experiments, as will be easily understood, have
furnished us with a great deal of valuable information,
which could not have been derived by studying the mix-
tures of pigments on the painter's palette. They supply us
with material which can be used in unraveling many col-
our-problems, presented by nature or art, which otherwise
would be quite beyond our grasp. The experiments them-
selves unfortunately are quite difficult, and for then- proper
execution require knowledge and skill as well as much pa-
tience. There is, however, another method of mixing coloured
light to which no such objections apply, for it is simple and
quite within the reach of all who are interested in this sub-
ject. We refer to the method of rotating disks which has
already once or twice been mentioned, f If a disk of card-
board be painted, as indicated in Pig. 47, with vermilion
and a bluish-green pigment, and then set in rapid rotation,
these colours will be mixed (in the eye of the observer),
and the whole disk will assume a new and uniform tint,
which will be that due to a mixture of the coloured light
sent out from the two halves of the disk (Fig. 48). When

■•' Or rather green-blue. \ See Chapter IX.

ON THE MIXTURE OF COLOURS. 131

we analyze this experiment, we find that what actually does
take place is this : At any one particular instant a certain
portion of the retina of the eye will be affected with red
light ; the disk then turns and presents to the same portion
of the retina bluish-green light ; then follows red, then
bluish-green, etc. Hence the retina is really acted on by
alternate presentations of the two masses of coloured light,
the intervals between these substitutions being something
less than one fiftieth of a second. Now, it so happens that
these alternate presentations have the same effect on the
eye as simultaneous presentations. This is not the least
valuable result of the spectral experiments above described.

Fio. 47.— Disk pfitated with Ver- Fia. 48.— Appearance presented

mlllon and Blue-green. by Eod and Blue-green Disk

when in Kapid Botation.

for it enables us by an easy method to pursue our colour-
investigations without having direct recourse to the spec-
trum. There is one respect in which the mixture by rotat-
ing disks actually does differ from that where the presenta-
tion is simultaneous. If we simultaneously present to the
eye two masses of coloured light, it is plain that the lumi-
nosity of the mixture will be equal to the sum of the lumi-
nosities of the two components (or at least must approximate
to it) ; thus, if the luminosity of our red light be 25, and
that of our greenish-blue 30, the luminosity of the tint of
mixture will be 55. If, however, these two masses of light
act on the eye alternately, as. is the case with rotating

132

MODERN CHROMATICS.

disks, the luminosity of the mixture-tint will be not the

sum but the mean of the separate luminosities ; that is, 271-.*

This method of mixing colours was mentioned in the

second century, in the "Optics" of Ptolemy. f It was

Fig. 49. — One of Maxwell's Bisks.

rediscovered by Musschenbroek in 1762, and finally greatly
improved by Maxwell. The last-named physicist modified
the disks, so that it became possible easily to mix the col-
ours in any desired proportion. This important improve-

FiGs. 50 and 61. -Two Views of a Fair of Maxwell's Disks in Combination.

ment is effected simply by cutting a slit through the disk
from the centre to the circumference, as indicated in Fig.
49. The slit enables the experimenter to combine two or
even more disks on the same axis, and to adjust them so

* Compare the results obtained by the author and giyen in Chapter HI.
f " Bibliographie Aiialytique," by J. Plateau (ISYT).

ON THE MIXT0IIE OF COLOURS. 133

that they present their respective surfaces in any desired
proportion. (See Figs. 50 and 51.) The relative propor-
tions of the two colours can then be obtained, as was done
by Maxwell, by placing a graduated circle around the
disks. The author finds it better to apply a graduated cir-
cle of pasteboard to the face of the disk, this circle being
made a little smaller than the disk itself ; the centring is
insured by its contact with the axis on which the disk is
fastened. (See Fig. 52.) Maxwell divided his circle into

Fig. 52.— Mode of measuring the Colours on the Disks.

100 instead of 360 parts ; this is convenient, and tenths of
a division can readily be estimated by the eye. There is
another very important feature connected with Maxwell's
disks : they can easily be arranged so as to furnish colour-
equations, which are of great use in chromatic studies.
For example, returning to our compound disk of red and
bluish-green, and remembering that these colours are com-
plementary, it is evident that, if we give to the red and
bluish-green surfaces the proper proportions, we can from
them produce white, or, what is the same thing, a pure
grey. But a pure grey can also be produced by rotating a
white and black disk similarly arranged. Hence, in an ex-
periment of this kind, we place on the axis, first, the disks
of vermilion, and bluish -green, and then on the same axis
smaller disks of black and white pasteboard, cut in a simi-
lar manner with radial slits. By repeated trials we can

134 MODERN CHROMATICS.

arrange tlie coloured disks so that they furnish a grey as
neutral and pure as that due to the black and white disks ;
and these last can be arranged so that the grey furnished
by them is as luminous as that given by the other two. In
an experiment of this kind it was found that to produce a
pure grey it was necessary to take 36 parts of vermUion
and 64 parts of bluish-green. This grey was in all respects
exactly matched by 21 "3 parts of white and 78*7 parts of
black. The disk when stationary presented the appearance
indicated in Fig. 58 ; when in rotation, that of a pure, uni-

FiG. 53. — Large Disk of Red and Blue-green arranged for the Frodaction of a Pure
Grey, Small Disk of Black and White furnishes the same Grey.

form grey. All this can be put in the form of an equation
by writing 86 red + 64 blue-green = 21-3 white -f 78-7 black.
We have here expressed the proportions in which it is
•necessary to take these particular colours for the produc-
tion of a grey ; the luminosity of this grey is also expressed
in terms of black and white paper. According to our equa-
tion, if we set the luminosity of white paper equal to 100
and that of black paper equal to nothing, then the lumi-
nosity of the grey will be equal to 21-3 per cent, of that of
white paper. It is not strictly true that the luminosity of
black paper is equal to nothing, or that it reflects no light
at all. Some careful experiments were made by the author
on this point, and the following result reached : If black
pasteboard is prepared by painting its surface with lamp-

ON THE MIXTURE OF COLOURS. 135

black in powder to which just enough spirit varnish has
been added to cause it to adhere closely, but not to shine,
then a uniform surface will be obtained, which reflects a
small but definite amount of the light falling on it. If, as
before, we set the luminosity of white pasteboard as 100,
then that of this kind of black paper will be 5'2 ; or, in
other words, it reflects about five per cent, as much light
as white, pasteboard. This knowledge enables us to correct
the equation just given ; instead of 21'3 white, we should
write 25 •4.

In the above example we have taken the case of two
complementary colours, and have obtained a measure for
the white or grey light which they furnish by mixture ;

Fio. S4.— Vermilioo and Emerald.ffraen Disks arranged to produce a Yellow by Rota-
tion. This yellow is imitated by small chrome-yellow, black, and white disks, as
arranged in the figure.

when the resultant tint is not grey, but some decided col-
our, we can in a similar way assign to it a numerical value.
Take the case of vermilion and emerald-green : Disks
painted with these colours can be made to furnish a whitish
yellow, as demanded by Young's theory, and we can ex-
press the value of this yellow in terras of chrome-yellow ;
that is, by darkening chrome-yellow and rendering it pale.
This we accomplish by combining chrome-yellow mth a
black and a white disk. As the result of an experiment
of this kind, the author obtained the equation : Verm.
51 -1- em. -green 49 = ch.-yel. 20 -I- white 8 + black 72.

136 MOI>BKN CHROMATICS.

The compound disk properly arranged is seen in Fig. 54.
The reader may be somewhat surprised to notice that it
was necessary to dull the chrome-yellow so greatly before
making it similar in colour to the yellow produced by mix-
ing the green and red light ; it must, however, be remem-
bered that chrome-yellow does not quite belong to the same
set of colours of which vermilion and emerald-green are
members ; that is to say, if we represented the red space of
a normal spectrum with vermilion, and the green space
with emerald-green, then chrome-yellow would be too
bright or luminous for the yellow space, and we should
have to substitute for it a less brilliant yellow pigment.

In -the same manner, with the aid of properly painted
disks, we can make a series of experiments on the mixture
of other colours, and satisfy ourselves of the correctness of
the results already given in this chapter. For instance, by
combining a yellow with a vermilion disk in various pro-
portions, we obtain a series of orange or orange-yellow hues,
which are as saturated in appearance as the original con-
stituents. Red lead with a yellowish-green disk gave a
fair yellow, and the same yellowish-green disk when com-
bined with vermilion furnished a fine orange or yellow,
according to the proportions. These corresp5nd to the re-
sults .given in Table I. In the same way those contained
in the other tables can be verified. Naturally, some care
must be exercised in the selection of the pigments with
which the disks are painted ; thus, the author finds that the
pure red of the spectrum can be imitated by vermilion
washed over with carmine. Vermilion itself corresponds
to the red space of the spectrum about half way between
C and D ; red lead answers for a red-orange situated nearer
still to D, etc. The parts of the spectrum which these and
other pigments represent are indicated in Chapter HI, to
which the reader is referred for further information.

In preparing a set of disks for accurate experiments, it
will be necessary of course to compare their colours care-

ON THE MIXTURE OF COLOURS.

137

fully with those regions of the spectrum which they are
intended to represent. This can be done with the aid of
the spectroscope in the method indicated in Chapter III.
A set of disks with carefully determined colours is quite
valuable, not only for experiments of this kind, but also to
enable us to produce a vast variety of tints at will, which
can be recorded and afterward accurately reproduced when
necessary.

"We pass on now to the description of a beautiful and
simple piece of apparatus contrived by Dove, for mixing
the coloured light furnished by stained glass, and called by
him a dichrooscope. This consists of a box 81 millimetres
long, 75 high, and 70 broad ; three sides are open, but can
be closed by opaque slides or by plates of coloured glass.
(See Fig. 55, which shows the box in perspective.) Fig. 56

Tis. 6S.— The Box of Dove's DiohroBsoope.
The plates of coloured glass are removed
and the three sides left open. The six
plates of window-glass are shown.

Fig. 56.— The Dichrooscope shown
in section.

is a vertical section,' in which G R and R D are plates of
coloured glass ; G P is an opaque slide made of blackened
cardboard, in which a square aperture has been cut ; P R
represents a set of six glass plates made from window-
glass of the best quality ; these are of course colourless.
At S S, Fig. 57, is a silvered mirror, and at N a Nicol's
prism. The action of the apparatus is as follows : Let us
suppose that G R is a plate of green glass, and R D one of
red ; then the light from the sky, striking on the mirror
SS, is reflected through RD and the. plates PR, and

138

MODERN CHROMATICS.

finaUy reaches the eye ; it will of course be coloured red.
But light from the sky also falls on the plate of green glass,
G R, penetrates it, is reflected from the glass plates at P R,
and also reaches the eye. The eye, then, wiU be simulta-
neously acted on by red and green light ; and, if the Nio-
ol's prism at N be removed, this mixture will be seen, but
we shall have no means of regulating the proportions of the
red and green light. But, by restoring the Nicol's prism
to its place, and rotating it, it is possible to mix the red and
green light in any desii-ed proportion.* "When the appa-

FiG. 5T. — The Dichrooscope as arranged for use.

ratus is provided with red and green glass as above indi-
cated, a dull yellow wUl sometimes be given without the
use of the Nicol's prism ; with its aid, this can always be
accomplished ; the yellow will pass into greenish-yellow or
orange, according as the proportions of the two constitu-
ents are varied. It is best to employ glasses the tint of
which is not too dark, as we do not readily recognize dark
yellow as such. The author easily obtained pieces of green

* Many beautiful experiments with polarized light can be made with
this little apparatus ; for an account of them the reader is referred to Pog-
gendorflf's " Annalen," ex., p. 265, or to the " American Journal of Science,"
Tol. xxxi., January, 1861.

ON THE MIXTURE OP COLOURS. 139

and purple glass ■which gave pure white ; yellow and blue
glass also did the same. If the hue of the yellow glass was
too deep, the white was always tinged pinkish. Red and
yellow gave orange ; green and yellow, yellowish-green ;
red and blue, purple. All these results are quite in accord-
ance with those obtained by mixing the coloured light of
the spectrum.

In the previous chapter we have described a method
which was contrived long ago by Lambert for mixing the
coloured light from painted surfaces (see Fig. 39, page 110).
The light from the blue paper is transmitted directly to the
eye, and that from the yellow paper reaches the eye after re-
flection ; their action is of course simultaneous. By moving
the two pieces of paper nearer together or farther apart, it
is possible to vary their apparent brightness, and thus to
regulate the proportion of blue and yellow light which
reaches the eye ; the yellow will predominate when the
papers are near together, the blue as they are moved fur-
ther apart. Chrome-yellow (the pale variety) and ultra-
marine-blue, when combined in this apparatus, give an
excellent white, and emerald-green and vermilion give a
yellowish or orange tint, according to the arrangement. It
is difficult to obtain a good representative of violet among
the pigments in use by artists ; the author finds that some
samples of the aniline colour known as " Hoffmann's violet
B B " answer better than any of the ordinary pigments.
If a deep tint of its alcoholic solution be spread over paper,
and combined in the instrument with emerald-green, a blue,
a greenish-blue, or a violet-blue can readily be produced.
It is evident that a multitude of experiments of this char-
acter can be made, the number of colours united at one
time being limited to two. The results of course agree
with Young's theory.

Another method of mixing coloured light seems to have
been first definitely contrived by Mile in 1839, though it
had been in practical use by artists a long time previously.

140 MODERN CHROMATICS.

We refer to the custom of placing a quantity of small dots
of two colours very near each other, and allowing them to
be blended by the eye placed at the proper distance. Mile
traced fine lines of colour parallel to each other, the tints
being alternated. The results obtained in this way are true
mixtures of coloured light, and correspond to those above
given. For instance, lines of cobalt-blue and chrome-yellow
give a white or yellowish-white, but no trace of green ;
emerald-green and vermilion furnish when treated iu this
way a dull yellow ; ultramarine and vermilion, a rich red-pur-
ple, etc. This method is almost the only practical one at the ■.
disposal of the artist whereby he can actually mix, not pig-
ments, but masses of coloured light. In this connection we
are reminded of an interesting opinion of Ruskin which has
some bearing on our subject. The author of " Modem
Painters," in his most admirable " Elements of Drawing,"
says : " Breaking one colour in small points through or
over another is the most important of all processes in good
modern oil and water-colour painting. ... In distant ef-
fects of a rich subject, wood or rippled water or broken
clouds, much may be done by touches or crumbling dashes
of rather dry colour, with other colours afterward put cun-
ningly into the interstices. . . . And note, in filling up
minute interstices of this kind, that, if you want the colour
you fill them with to show brightly, it is better to put a
rather positive point of it, with a little white left beside or
round it, in the interstice, than to put a pale tint of the col-
our over the whole interstice. Yellow or orange will hardly
show, if pale, in small spaces ; but they show brightly in
fine touches, however small, with white beside them."

This last method of mixing coloured light is one which
often occurs in nature ; the tints of distant objects in a
landscape are often blended in this way, and produce soft
hues which were not originally present. Even near objects,
if numerous and of small dimension, act in the same man-
ner. Thus the colours of the scant herbage on a hillside

ON THE MIXTURE OF COLOURS. 141

often mingle themselves in tMs way with the greenish-grey
tints of the mosses and the brown hues of the dried leaves ;
the reddish- or purplish-brown of the stems of small bushes
unites at a little distance with their shaded green foliage ;
and in numberless other instances, such as the upper and
lower portions of mosses, sunlit and shaded grass-stalks, and
the variegated patches of colour on rocks and trunks of
trees, the same principle can be traced.

There is another mode of mingling coloured light, which
is not much used by physicists, though it is of constant oc-
currence in nature. We refer to the case where two masses
of coloured light fall simultaneously on the same object.
Sunsets furnish the grandest examples of these effects, the
objects in a landscape being at the same time illuminated
by the blue sky and the orange or red rays of the sinking
sun. Minor cases happen constantly ; among them the
commonest is where a coloured object reflects light of its
own tint on neighboring objects, thus modifying their hues
and being in turn modified by them. The white or grey
walls of a room are often very wonderfully tinted by col-
oured light which is cast on them in nebulous patches by
the carpet, window-curtains, or other coloured objects that
happen to be present. In all cases where the surface re-
ceiving the manifold illumination is white or grey, or but
slightly coloured, the laws for the mixture of coloured
light which have been explained above hold good ; when,
however, this surface has a distinct colour of its own, the
phenomena are modified in a manner which will presently
be noticed.

We pass on now to compare the results which are ob-
tained by mixing coloured lights with those which are
given by the mixture of coloured pigments. It was for a
long time supposed that these were identical, and that ex-
periments on mixtures of coloured light could be made
with the aid of the painter's palette. Lambert appears to
have been the first to point out the fact that the results in

142 MODERN CHROMATICS.

the two oases are not always identical. Thus the celebrated
experiment of combining blue with yellow light, and ob-
taining not green but white, was first made by him with the
apparatus shown in Fig. 39, page 110. The same fact was
afterward independently discovered by Plateau, and finally
by Helmholtz, who, using it as a starting-point, made an ex-
amination of the whole subject. When we study this matter
with some attention, we find that in mixing pigments two
different effects are produced. Suppose we mix chrome-
yellow and ultramarine-blue, both in dry powder. If we
rub this mixture on paper, we shall produce a uniform and
rather dull green. An examination even with a moderately
powerful microscope will fail to reveal the separate parti-
cles of the two pigments. Yet we know that there must
be a superficial layer, made up of a mosaic work of blue
and yellow particles placed side by side. These two sets of
particles send light of their own colour to the eye, which
there undergoes a true mixture, and gives as the resultant
hue a yellowish-grey.- Thus far the result entirely corre-
sponds with that produced by mixing two niasses of col-
oured light. The second and more important effect is
brought about by light which penetrates two or more lay-
ers of particles. Here the light undergoes absorption in
the manner explained in Chapter VII. : the yellow particles
absorb the blue and violet ; the blue particles, the red,
orange, and yellow rays. The green light is absorbed also
by both sets of particles, but not nearly so much as the
other rays. From all this it follows that chrome-yellow
and ultramarine-blue jointly absorb all the colours which
are present in white light, except green ; hence green light,
as the result, is reflected back from the surface, and reaches
the eye of the observer. This green light is finally mingled
with the yellowish-grey light before mentioned. When dry
pigments are employed, both of the effects just described
are always present. If the pigments are used as water-
colours, the light from the surface is diminished, and with

ON THE MIXTURE OF COLOURS. I43

it the first of these effects ; and a still further diminution
of it takes place if the pigments are ground up in oil.
From all this it is evident that by mingling two pigments
we obtain the resultant effect of two acts of absorption due
to the two pigments : white light is twice subjected to the
process of subtraction, and what remains over is the col-
oured light which finally emerges from the painted surface.
On the other hand, the process of mixing coloured light is
essentially one of addition ; and, this being so, we find it
quite natural that the results given by these two methods
should never be identical, and often should differ widely.
From this it follows that painters can not in many cases
directly apply knowledge acquired from the palette to the
interpretation of chromatic effects produced by nature, for
these latter often depend to a considerable extent on the

Fig. 58.— Two Apertures in Black Cardboard covered with Eea and Green Glass
(natural size.)

mixing of masses of differently coloured light. This fact is
now admitted in a general way by intelligent artists, but
probably few who have not made experiments in this direc-
tion fully realize how wide are the discrepancies which exist
between the results given by the two different modes of
mixture. A few years ago Dove described a method of
studying this subject with the aid of stained glass ; and, as
it would be difficult to devise a simpler or more striking
mode of making such experiments, we will give his process
in full.

144

MODERN CHROMATICS.

In a blackened piece of cardboard two apertures are
cut, about a third of an inch broad, as indicated in Fig. 58.
Over these apertures are fastened pieces of stained glass —
for instance, red and green ; light from a white cloud is
then made to traverse the glasses. At P, Pig. 59, is an

llllllllllilllllllllillilllllii!IIIIMIIMiBi:iiaililiilij.,i;:ill

Fig. 59.— Mode of using DoTe'8 Apparatus. E G is cart

at P is the prism of cale spar.

red and greeB glass ;

achromatic prism of calc spar, which doubles each of the
little patches of coloured light, so that the observer on
looking through the; prism actually sees two red images of

Fis. 60.— The Appearance presented when Eed and Green Light are mixed by Bove's

Apparatus.

exactly equal brightness, and also two similar green images.
Now, by revolving the calc-spar prism the experimenter
can cause one of the red images to overlap one of the
green, and thus it is possible to mingle the red and green

ON THE MIXTURE OF COLOURS.

145

light from the coloured glasses. In one experiment made
by the present writer the colour of this miztui-e was orange
(see Fig. 60). Upon removing the glasses from the instru-
ment, placing them one over the other and allowing white
light to pass through them, the effect due to the double
absorption manifested itself ; but now the colour of the
transmitted light was not orange, or even brown, but dark
green. If these two glasses had been ground to powder,
mixed with oil, and then the mixture painted on canvas, it
would have exhibited not an orange but a dark-green hue.
We give below the results of a set of experiments which
were recently made by the writer, and which illustrate the
differences to be encountered in the two modes of proceed-
ing : .

Resdlts given bt Dove's Apparatus.

Ooloura of Glass.

Result obtained by mixture
of Light.

Result obtained by A.b-
soi-ption.

Red and green

Orange.
Pale yellow.

Dark green.

*Red and green

Black.

Yellow and blue

White.

Fine green.

*Yellow and blue

Pinkish-white.

Fine olive-green.

Violet-purple.
Yellow.

Deep red.

Yellow and dark purple . . .

Deep orange.

*Yellow and dark purple. .

Pale orange.

Dark brown.

Purple and green

White.

Dark green.

Yellow and red

Yellow, slightly orange.
Orange.

Deep orange-red.

*Yellow and red

Red.

Yellow and bluish-green. . .

Yellow.

Yellowish-green.

*Yellow and blue-green.. . .

Yellowish-white.

Rich yellowish-green.

**Yellow and blue-green . .

Pale greenish-yellow.

Olive-green.

Purple and blue-green. . . .

Pale blue-green.

Dark violet.

Purple-violet and green . . .

Pale violet-blue.

Black.

These are not experiments selected so as to show wide dif-
* Sample more deeply tinted.

146 MODERN CHROMATICS.

ferences ; they comprise the entire set that was made on
the occasion referred to, and are simply transcribed from
the author's note-book. Yet it will be seen that in not a
single case do the two methods furnish the same result ;
and as a general thing they differ so entirely that it would
be quite impossible even to predict the nature of one of the
sets of tints from a knowledge of the other. These experi-
ments, then, exhibit the wide difference between the effects
produced by mixture of light and the absorption of light ;
but, as has already been remarked, when pigments are
mixed on the palette, the resultant hue depends partly on
the process of true mixture and partly on that of absorp-
tion, the latter of course predominating. Hence the results
in the table, though instructive, are not necessarily strictly
applicable to the painter's palette, which is best studied by
another method.

Fig. 61.— Disk for showing the Difference bebveen mixiiig Coloured Light and Coloured
Pigments. The outer disk is painted with the pure pigments, the small disk with a
mixture of the same pigments.

For an examination of this matter the author adopted
the following mode of proceeding : Two tolerably deep
washes of water-colour pigments were prepared — for in-
stance, vermilion and ultramarine-blue — with which two of
Maxwell's disks were separately painted. Afterward an
equal number of drops of the same washes were mingled
on the palette, and a third and smaller disk painted with this
mixture. The disks were placed on a rotation apparatus,

ON THE MIXTURE OF COLOURS. I47

arranged as in Fig. 61, the vermilion and ultramarine cover-
ing each one half of the larger disk ; the smaller one, ex-
hibiting the result furnished by the palette, being placed in
the centre. When the compound disk was rotated, the col-
ours of its outer portion underwent true mixture, and it was
easy to compare the resultant tint with that furnished by
the palette. In the experiment referred to the result was
as follows : The larger disk became tinted red-purple,
alongside of which the smaller disk seemed grey, so duU
and inferior was its colour. The real colour of the smaller
disk was a dull violet-purple. It will be noticed, then, that
not only was the colour much darker and less saturated, but
it had been moved from a red-purple to a violet-purple.
Next, in order to ascertain how^much the pigments had
been darkened by mixture on tlhe palette and otherwise
changed, a black disk was combined with the vermilion and
ultramarine disks, and various amounts of black introduced
into the red-purple mixture by rapid rotation. It was found
impossible in this way to bring the colour of the larger disk
to equality with that of the smaller one, it remaining always
too saturated in hue. Some white was then added to the
large disk, and equalization finally effected. It was then
found that twenty-one parts of vermilion, twenty parts of
ultramarine, with fifty-one parts black and nine parts white,
made a tint by rotation which was identical with that given
by mixing up the vermilion and ultramarine on the palette.
The large amount of black which it was necessary to add
strikingly illustrates the general proposition that every
mixture of pigments on the painter's palette is a stride
toward blackness. We give now the results of the other
experiments :

148

MODEKN CHROMATICS.

Table showing the Effects of mixing Pigments by Rotation and on
THE Palette.

Pigments.

Violet (" Tiolet-carmine ") (

Yellow-green (Hooker's green) j'

Violet ("violet-carmine") [

Yellow (gamboge) j

Violet (''violet-carmine") )

Green (Prussian-blue and gamboge). . . J

Violet (" violet-carmine") )

Prussian-blue j

Violet ("violet-carmine") (

Carmine j'

Gamboge

Prussian-blue

Carmine i

Hooker's green j

Carmine |

Green f

By Rotation.

Yellowish-grey.

Pale yellowish-
grey.

Greenish-grey.
Blue-grey.

Pink-purple.

Pale greenish-
grey.

Yellowish-or-
ange (flesh-tint).

Pale reddish
(flesh-tint).

On the Palette.

Brown.

■grey.

Grey.

Blue-grey.

Dull red-purple.

Full blue-green.

Brick-red.

Dark-red.

It will be noticed that in only one case do the results of the
two methods coincide ; in all the others the tints from the
palette are not only much darkef, but also different. Col-
our-equations were then obtained for the eight cases above
given in exactly the manner indicated for vermiUon and
ultramarine-blue ; and as they present the facts in an exact
manner, showing how much black it was necessary to intro-
duce, and how far the proportions of the two component
colours had to be varied, they are given below :

Mixture on Palette.

60 violet + 60 Hooker's green. .

Mixture by Rotation.
21 violet -I- 22-5 Hooker's green -f 4
vermilion + 62-5 black.

60 violet + 60 gamboge =64 violet + 20 gamboge + 26 black.

60 violet -f 50 green =60 violet -|- 18 green + 32 black.

60 violet + 50 Prussian-blue. . . = 47 violet + 49 Prussian-blue + 4 black.

ON THE MIXTUKE OF COLOURS. I49

60 violet + 50 carmine =\^^ '■'°'^' + ^^ "^""^^ + ^ ^l*™""^-

( rine + 19 black.

50 gamboge + 50 Prussian-blue. = \ ^^ r"""" gamboge) + 42 Prussiar-

/ blue + 41 green + 4 black.

»f, ■,- . »„ ,^ . * 21 vermilion + 20 ultramarine + 51

50 vermilion + 50 ultramarme. . = ^ ' » uc -r uj.

( black + 9 white.

„„ XT , , _. . I 23'5 yellow-green (Hooker's green) + 8

60 Hooker's green + 50 carmine = < j _ b \ s =■;"; -r

l carmine + 52 vermilion + 16 black.
50 carmine + 50 green =50 carmine + 24 green + 26 black.

It will be noticed that the amount of black which it was
necessary to introduce, in order to darken the true mixture
of the colours so as to match the mixture of the pigments,
was a very variable quantity, ranging from four to fifty-two
per cent. It is for this reason that artists are so careful in
their selection of pigments for the production of definite
tones, particularly when they are to be luminous in quality.
In four of these experiments it was found impossible to
bring about equality without adding to the two original
constituents a third colour, and in one case white had to be
added ; so that, in more than half the cases examined, the
original colours were found incapable of reproducing by a
true process of mixture the tint obtained on the palette
without the aid of a foreign element. These experiments
serve, then, to show that the results furnished by the pa-
lette can not be relied on to guide us in the interpretation
or study of effects in nature depending on the mixture of
coloured light.

We propose now to consider the results which are pro-
duced when a coloured surface is exposed to a coloured
illumination and at the same time to white light. Effects
of this kind are very common in nature, and are frequently
purposely selected by artists as themes ; in a minor degree
they are always present to some extent, even when we seek
to avoid them. With the knowledge which we have now
gained, it is possible for us to recognize the fact that in
such cases the resultant tint of the surface will depend on

150

MODERN CHROMATICS.

three circumstances : first, on the colour which it assumes
owing to the presence of the white light — that is to say,
which it has owing to its natural or, as artists call it, " local
colour " ; secondly, on the colour communicated to it by
that portion of the coloured light which is reflected unal-
tered from its surface ; and to these there must be added,
thirdly, the effects produced by the coloured light which
penetrates below the surface, and is reflected after under-
going a certain amount of absorption. It is quite easy to
make satisfactory experiments on this matter with the aid

Fio. 62.— White light from a window ftlls on stained glass at 6, and is then concentrated
by the lens on a sheet of white or coloured paper at P.

of a simple arrangement contrived by the author. At a
distance of some eight or ten feet from a window, a lens,
with a focal length of about five inches, is placed on a
table, in such a way as to concentrate the white light from
the window. In front of the lens a plate of coloured glass
is held, and the result is that we obtain a bright beam of
coloured light, which can be thrown on any coloured sur-
face, such, for example, as painted paper (see Fig. 62). If

ON THE MIXTURE OF COLOURS. 151

the walls of the room are white, the paper will at the same
time be exposed to a white illumination ; and, by turning it
or removing it farther from the lens, the proportions of this
double illumination can be varied at wiU. We will describe
two experiments that were made with this arrangement :
Yellow light was obtained by using a plate of glass which
transmitted light having to the eye a pure yellow hue,
without any tendency to orange-yellow or greenish-yellow.
In this beam of light a piece of paper, painted with a very
intense, deep hue of artificial ultramarine, was held. The
portion illuminated by the yellow light appeared almost
quite white, showing that a true mixture of the colours had
taken place. It is well known that it is difficult to decide
about the actual colour of a spot when it is surrounded by
a coloured field ; hence, in order to avoid deception by con-
trast, it is well in these experiments to observe the spot_
which has received the double illumination through an
aperture cut in black paper, which is to be held in such a
way as to permit a view only of this spot. This precaution
was taken in the present case, and also in all the experi-
ments that are given below. The ultramarine paper was
then removed, and its place supplied by some which had
been painted with Prussian-blue. The spot now appeared
of a bright green colour, which proved that an action had
taken place similar to that produced by mixing pigments
on the palette. The explanation is as follows : The yellow
glass transmits yellow, green, orange, and red light ; and,
as was explained in the previous chapter, these lights taken
together make a light which appears to us yellow. That
portion of this compound yellow light which penetrates the
Prussian-blue undergoes a process of absorption ; the green
constituent, however, is not absorbed, and consequently is
reflected rather abundantly from the paper. But some of
the yellow light is reflected unaltered from the immediate
surface of the paper ; this mixes with the blue light (due to
the white illumination), and makes white ; so that what we

152 MODERN CHROMATICS.

finally have is green mixed witli more or less white. In
the experiment where the ultramarine paper was used, no
doubt some absorption took place, but it was not sufficient
to modify the result materially ; the blue and yellow light
simply united, and formed white light. Below are given,
in the form of tables, a large series of experiments made
recently by the author ; and an examination of them will
show that for the most part the resultant tint depends
rather on a true mixture of coloured lights, and that absorp-
tion acts only as a minor agent in modifying the results :

Table I.

Yellow Light falliDg on
Paper painted with

Carmine gave Red-orange.

Vermilion gave Bright orange-red.

Orange * gave Bright orange-yellow.

Chrome-yellow gave Bright yellow.

Gamboge gave Bright yellow.

Yellowish-green f gave Yellow.

Green J gave Bright yellow-green.

Blue-green § gave Yellow-green (whitish).

Cyan-blue | gave _ Yellow-green.

Prussian blue gave Bright green.

Ultramarine-blue gave White.

Violet ^ gave Pale reddish tint.

Purple- violet ** gave Orange (whitish).

Purpleff gave Orange.

Black a gave Yellow.

* Mixture of red lead and Indian- yellow.

f Mixture of gamboge and Prussian-blue.

ij: Mixture of emerald-green with a little chrome-yellow.

§ Mixture of emerald-green with a little cobalt-blue.

II Mixture of cobalt-blue and emerald-green.

^ Hoffmann's violet B. B.

** Hoffmann's violet B. B. and carmine.

■)-f Hoffmann's violet B. B. and carmine.

J:j: Lampblack.

ON THE MIXTURE OF COLOURS. 153

Table II.
Red Light falling on
Paper painted with

Carmine gave Red.

Vermilion gave Bright red.

Orange gave Red-orange and scarlet.

Chrome-yellow gave Orange.

Gamboge gave Orange.

Yellowish-green gave Yellow and orange.

Green gave Yellow -and orange (whitish).

Blue-green gave Nearly white.

Cyan-blue gave Grey.

Prussian-blue gave Red-purple or blue-v/olel.

Ultramarine-blue gave Red-purple or blue-violet.

Violet gave Red-purple.

Purple-violel gave Red-purple.

Purple gave Purple-red or red.

Black gave Dark red.

Table III.

Green Light falling on
Paper painted with

Carmine gave Dull yellow.

Vermilion gave Dull yellow or greenish yellow.

Orange gave Yellow and greenish-yellow.

Chrome-yellow gave . . . . Yellowish-green.

Gamboge gave Yellowish-green.

Yellowish-green gave . . . Yellowish-green.

Green gave Bright green.

Blue-green gave Green.

Cyan-blue gave Blue-green.

Prussian-blue gave Blue-green, cyan-blue.

Ultramarine-blue gave . . Cyan-blue, blue.

Violet gave Cyan-blue, blue, violet-blue (all whitish).

Purple-violet gave Pale blue-green, pale blue.

Purple gave Greenish-grey, grey, reddish-grey.

Black gave Dark green.

Table IV.

Blue Light falling on
Paper painted with

Carmine gives Purple.

Vermilion gives Red-purple.

Orange gives Whitish-purple.

154 MODERN CHROMATICS.

Chrome-yeUow gives Yetlowish-grey, greenish-grey.

Gamboge gives Yellowish-grey, greenish-grey.

Yellowish-green gives Blue-grey.

Green gives Blue-green, cyan-blue.

Blue-green gives Cyan-blue, blue.

•Cyan-blue gives Blue.

Prussian-blue gives Blue.

Dltramarine-blue gives Blue.

Violet gives Ultramarine, violet-blue.

Purple-violet gives Blue-violet.

Purple gives Violet-blue, purple- violet.

Black gives Dark blue.

These experiments, taken as a whole, show that, in calcu-
lating for the eifects produced by illuminating coloured
surfaces by coloured light, we must be guided mainly by
the laws which govern mixtures of coloured lights, rather
than by those which can be deduced from experience with
pigments ; they are certainly useful in teaching us, when
studying from nature, fearlessly to follow even the most
evanescent indications of the eye, utterly regardless of the
fact that they disobey laws which we have learned from
the palette.

We pass on now to consider the changes in tint which
take place when coloured surfaces are illuminated by lamp-
light or gas-light. If we undertake to make experiments in
this direction simply by viewing coloured surfaces by lamp-
light in a room illuminated with it, correct results can not
be obtained ; for by this very method we have practically
rendered ourselves colour-blipd to a certain extent, and
have become incapable of judging correctly of quite a series
of hues. Gas-light is deficient in the violet, blue, and blu-
ish-green rays ; hence its resultant tint is not white, but
orange-yellow. If we are immersed in this light, it will
appear to us white, and our judgment of all colours will be
more or less disturbed : yellow surfaces will appear white
or whitish ; blue surfaces, more greyish-blue, or, if pale,

ON THE MIXTURE OF COLOURS.

155

even pure grey. The actual changes effected by artificial
illumination may readily be studied by the following sim-
ple method, contrived by the author : A camera-obscura is
placed in a room illuminated by ordinary daylight ; in
front of it, at a short distance, is placed a gas-flame or lamp-
flame, in such a way that the lens of the camera is capable
of forming an image ' of it of about half the natural size.
(See Fig. 63.) This image is now allowed to fall on col-
oured stuffs or on coloured paper placed behind the lens of
the camera at S ; it can be viewed, as indicated, through

:»/,////>

Fig. 63. — Light from Gas-flame is concentrated by Lens of Camera and falls on Coloured

Paper.

the top of the camera, and the resultant tint noted. In
the experiments made by the author a gas-flame was em-
ployed, along with a set of painted disks, representing the
principal colours. The disks, fourteen in number, were the
same as described in the following chapter, and constituted
together seven pairs, the colours of which were comple-
mentary, two and two. Below are the results :

1. A cai-mine disk when illuminated by the gas-flame
assumed an intense red hue, even more brilliant than by
daylight ; the complementary disk, painted blue-green,
appeared of a yellowish-green, not saturated, but rather pale.

156 MODERN CHROMATICS.

2. Vermilion appeared of an intense fiery red ; its com-
plement, green-blue, lost in strength, and became yellowish-
green and rather pale.

3. Orange appeared brilliant ; cyan-blue, the comple-
ment, became greenish-yellow and lost in saturation.

4. Yellow became brilliant, showing a tendency toward
orange ; its complement, blue, appeared white, or rather
pure grey. On the same occasion disks painted with
chrome-yellow were examined : two of them were rendered
somewhat orange-yellow ; the third was brought to almost
a full orange hue by the gas-light. A disk painted with
gamboge-yellow acquired something of an orange tint under
the gas-light.

5. Greenish-yellow was brought to a pure yellow ; its
complement, artificial ultramarine-blue, appeared violet.

6. Greenish-jello'w became pure yellow ; its comple-
ment, violet, was converted into a strong red-purple.

7. Full green passed into a bright, strong yellowish-
green ; its complement, purple, assumed an intense pm'-
plish-red hue, displaying less blue than by daylight.

These are the actual changes produced by the artificial
illumination as they appeared to an eye placed in ordinary
daylight, and consequently able correctly to note the sev-
eral tints. When the disks were examined at night by gas-
light, in many cases a different result was reached. The
carmine and vermilion disks still appeared very brilliant,
the tint in the case of the former being a pure red, while
the vermilion showed a tendency to red-orange. The orange
disk seemed to be changed in tint to a redder orange hue ;
the yellow, on the contrary, appeared paler. The greenish-
yellow disks did not show much change. The full green
was intense, appearing perhaps more bluish than by day-
light. Blue-green was liable to be confused with blue,
cyan-blue and blue with green ; artificial ultramarine-blue
appeared more purplish than by daylight ; violet became
purple, and purple a very red purple. Some other disks

ON THE MIXTURE OF COLOURS. 157

were also examined on this occasion : gamboge and chrome-
yellow showed a loss in saturation, looking whitish ; indigo
appeared dull greenish-grey ; Prussian-blue was confused
with blue-green ; genuine ultramarine-blue still was always
blue with a slight tendency to purple ; cobalt-blue exhib-
ited this same tendency, which reached a maximum in
French-blue. All the blues appeared much duller and
greyer than by daylight.

By comparing these two sets of experiments, it will be
seen how greatly the judgment of colour was influenced by
the circumstance that the prevailing illumination was yel-
low, and that hence a certain shade of yellow stood for
white, and gavo' a false standard to which all the colours
were referred. This was particularly noticeable in the case
of the yellow disks ; in point of fact, as the first set of ob-
servations showed, they reflected to the eye much yellow
light, and, as far as the mere physical action went, ought
to have produced the sensation of a strong, briUiant yellow
hue ; but, as all surfaces which professed to be white were
really (owing to the gas-light) yellow, this competition
caused the yellow disks to appear pale. Another case illus-
trates this disturbed judgment even better. In the first set
of experiments it was found that the blue disk when illu-
minated by gas-light really assumed a pure grey hue with-
out any trace of blue ; but at night, although it must have
sent to the eye this same pure grey light, it always appeared
either blue, greenish-blue, or bluish-green ; in other words,
the blue disk, when held near the gas-flame, sent to the eye
white light, which appeared blue, by contrast with the pre-
vailing yellow illumination. It is hardly necessary to add
that these causes affect our 'judgment of paintings and dec-
orations at night to a very considerable degree, the blues
being rendered less conspicuous, the blue-greys being mostly
abolished, and the yellows losing in apparent intensity.
Genuine ultramarine-blue is less affected than the other
blues, cobalt and artificial ultramarine-blue becoming pur-

158 MODERN CHROMATICS.

plish, and Prussian-blue quite greemsh. It hence follows
that paintings in which the blue tones are rather overdone
appear often better by gas-light ; but this is hardly the case
when the green hues are of somewhat too great strength,
the evil seeming often to be exaggerated by artificial illumi-
nation, which must of course be due to an act of the judg-
ment, as the greens really assume a more yellowish appear-
ance by gas-light or lamp-light, as was proved by the first
set of experiments. From this it follows that, if the chro-
matic composition of a picture is quite right for daylight,
it will be more or less wrong when viewed by gas-light ;
hence it would be desirable to illuminate picture galleries
at night with some kind of artificial lohite-light, a problem
which the future will no doubt solve.

All the appearances which have thus far been considered
could be satisfactorily observed and studied by a person
possessed of only a single eye. Let us now turn our atten-
tion for a moment to some very remarkable phenomena
which occur when different colours are presented to the
right and left eye. This is a case which happens occasion-
ally, particularly when we look at the reflection from pol-
ished surfaces or from water. In order to simplify matters,
let us take a case where, for instance, yellow light is pre-
sented to the right and blue light to the left eye. It is
very easy to make an experiment of this kind with the aid
of the stereoscope. Selecting one of the common paper
slides, we colour it as indicated in Fig. 64, and then view
it with the stereoscope. We have already seen that blue
and yellow light when presented to the same eye undergo
mixture on the retina and produce the sensation we call
white. This would lead us very naturally to suppose that,
if blue light were presented to the right eye and yellow to
the left, -the two sensations would be united in the brain
and would call up that of white. The effect is, however,
of a much more complicated character. Viewed in the

ON THE MIXTURE OF COLOURS.

159

stereoscope the figure will appear at one momeiit blue,
then yellow, as though it had no permanent colour of its
own ; sometimes, again, the observer seems to see one col-
our through the other, and is distinctly conscious of the
presence of both occupying apparently the same place, thus
giving rise to the idea that the object might have at the
same time two distinct colours. Meanwhile the little draw-
ing assumes a highly lustrous appearance, as though it were
made of polished glass ; this is quite beautiful, and strikes
with some astonishment those who see it for the first time.
After some little practice has been gained, the blue and
yellow colours will melt into a lustrous blue-grey or pure
grey tint now and then for a few seconds, when again the

YEL.

/
Y.

BLUE

YELLOW

Y£L.

BLUE

Fig. 64.— Slide for the Stereoscope, the Eight and Left Hand being differently coloured.

contradictory and confusing phantoms just mentioned will
make their appearance. Taken altogether, the effect is
quite wonderful, and suggestive of something like a new
sensation. There has been a good deal of controversy as
to whether a true blending or mixture of the two colours
actually does take place in the brain. The experiments of
De Haldat and Dove, and afterward of Lubeck, Foucault,
and R6gnault, all point to this result. The results obtained
by the author are also favorable to this view. But it must
be confessed that the mixture obtained by this method dif-
fers in one respect from those previously described. For,
when coloured light is mixed with the aid of rotating disks
or by Lambert's method, we see only the resultant tint, the

160 MODERN CHROMATICS.

two components disappearing entirely to give place to it.
On the other hand, in this binocular mixtui-e of colours, the
presence of each of the original colours is all the while to
some extent felt, and we are disposed to say that we see a
neutral or grey hue which has evidently been made out of
blue and yellow. Careful experiments by the author
' proved that the tint of the true mixture often differed from
that obtained by the use of the stereoscope ; colours which
were pale, however, united more readily than intense ones,
and gave less divergent results.* The binocular mixture of
colours always produces more or less lustre ; it is not even
necessary to employ distinct colours, the same effect being
brought about by the mixture of a light and dark shade of
the same colour, or simply by the binocular union of white
and black, as was shown by Dove. The lustrous appear-
ance of waves, ripples, and broken reflections in water is in
each case mainly produced in this way, and hence, strictly
speaking, can not be imitated by artists, who are necessarily
obliged to present the same colours, the same light or dark
shades, impartially to both eyes. It is for reasons similar
to the above that a somewhat lustrous appearance is com-
municated to an oil painting by varnish, or to a water-
colour drawing by glass ; the eye sees the picture through
the light slightly reflected from the glass or varnish, and is
enabled apparently to penetrate beneath the mere surface
of the pigment, and this slight illusion falls in with and
helps the design of the artist.

* " American Jburnal of Science," May, 1865.

CHAPTER XI.

COMPLEMENTARY COLOURS.

In the previous chapter we found that the mixture of
two masses of coloured light in some cases produced white
light ; this was, for example, true of mixtures of ultrama-
rine-blue and yellow, or of red and greenish-blue. Any
two colours which by their union produce white light are
called complementary. An accurate knowledge of the
nature and appearance of the complementary colours is
important for artistic purposes, since these colours furnish
the strongest possible contrasts. The best, in fact the
only, method of becoming acquainted with the appearance
of colours which ai-e complementary is by actually studying
them with the aid of suitable apparatus. The results thus
obtained should be at the time registered, not in writing, but
by imitating as far as possible the actual tints with brush
and palette. By the aid of polarized light it is possible to
produce with ease and certainty a large series of colours
which are truly complementary. There are quite a number
of instruments for accomplishing this, but perhaps the sim-
plest and best is that which was contrived by Brtlcke for
this express purpose, and called by him a schistoscope. (See
Fig. 65.) This little apparatus is merely a combination of
a low-power simple microscope with a polariscope, and can
easily be constructed. Starting from below, P is a piece of
white cardboard, which is fastened to the stand as indicated,
and is consequently capable of being turned so as to reflect
upward more or less white light, as may be required. N" is

162

MODERN CHROMATICS.

a Nicol's prism, whicli polarizes the light thus reflected ; it
is attached to a blackened stage, S. At A is a smaU square
aperture two millimetres in size. C is a crystal of calc
spar ; L is a convex lens of a focus such as to cause the two
images of the square opening furnished by the calc spar
just to touch each other. G and G are polished wedges of
glass, the angles being 18° ; for rough experiments they
may be dispensed with. In order to use this apparatus,

Fig. 65. — The Schistoscope, for the production of Complementary Colours. (Brucke )

the tube containing the calc spar is to be moved tUl distinct
vision is obtained of the square opening in the stage by
the eye placed at L, or rather of the two square openings
which will be seen ; the tube is then to be revolved till one
of these images disappears entirely, and is to be left in this
position. Besides the instrument it is necessary to provide
a large number of thin slips of selenite or crystalized sul-
phate of lime. If a clear transparent piece of this sub-
stance is procured, it will be easy with a penknife to split
off two or three hundred thin slips, and then with the
aid of the instrument to select those which are worth pre-

COMPLEMENTARY COLOURS. 163

serving. To observe the colours it is only necessary to lay
one of the slips on the stage between the calc-spar prism
and the Nicol's prism, and then to turn the selenite till two
brightly coloured squares are seen, as is indicated in Fig.
66. These two squares will always have colours which are

Pra. (36.— Complementary Colours as exhibited by the Bchlstoscope of Brucke.

complementary. The object of preparing a large number
of the slips of selenite is the production of a large series of
complementary tints. The thinner slips furnish colours
that are more saturated ; those which are thick give pale
colours, or colours mixed with much white light. It will
be found in this way that the following pairs of colours are
complementary :

Table of Complementary Colours.

Red Green-blue.*

Orange Cyan-blue.

* Following Helmholtz, most writers give bluish-green as the comple-
ment to red. These obsei-vations of Helmholtz were made on the spectrum,
the field being small and only a single eye employed. Extended observa-
tions with coloured disks, the hue of which can be studied in a more natu-
ral way and with both eyes simultaneously, have convinced the present
writer that the complement of vermilion is a very green blue, and even the
complement of carmine is a very green blue rather than a blue-green.

164 MODERN CHROllATICS.

Yellow Ultramarine-blue.*

Greenish-yellow Violet.

Green Purple.

In Fig. 67 these complementary colom-s are arranged in
a circle. They are of course only a few of the pairs that
can be noticed. The tints situated between red and orange
will have complements lying between greenish-blue and
cyan-blue ; those between orange and yellow, again, will
find complements between cyan-blue and ultramarine-blue,
etc. As before remarked, it is a good plan to copy the
results with water-colours ; this fixes the facts in the mem-
ory far better than mere momentary inspection.

~~~~^N

y \

RED

\.„^ Q.BLUE

Fig. 67. — Complementary Colours arranged in a Circle.

To study the complementary colours with the aid of the
spectrum is a much more troublesome process ; still, it is of
interest for us to know that the results are the same as
when polarized light is employed. Another point now de-
serves consideration. It might be supposed that the lumi-
nosity or apparent brightness of colours which are comple-
mentary would be the same, but this is far from being true ;
yellow, for example, is much more luminous than its com-
filementary blue, and the difference between greenish-yellow
and violet is still greater. Helmholtz, using the pure col-

* The oomplement of genuine ultramarine-blue is yellow, that of arti-
ficial ultramarine being a greenish-yellow. The artificial pigment, or
French-Wue, is a riolet-blue.

COMPLEMENTARY COLOXIRS. 165

ours of the spectrum, ascertained tliat the order of the
luminosities of the complementary colours is about that
given in the following table :

Yellow.

Orange and green about the same.

Red and cyan-blue about the same.

XJltramarine-blue.

Violet.

From this it follows that a violet which appears to the
eye quite dark is able to balance a bright greenish-yellow,
and form with it white, and the same is true of ultrama-
rine-blue and yellow ; red and its complement green-blue
have about the same, luminosities ; orange is somewhat
brighter to the eye than its complement cyan-blue. There
is another way of stating these facts : we can say that in
mixtures violet has a greater power of saturation than any
of the colours ; next follows ultramarine-blue, then red and
cyan-blue, etc.

The method of studying complementary colours with
the aid of polarized light and plates of selenite is simple and
beautiful, but there are many cases which it does not reach ;
above all, it fails to furnish us with the means of ascertain-
ing the complementary tints in just those instances which are
of particular interest, viz., the pigments. This happens be-
cause the colours furnished by the plates of selenite are for
the most part quite like those of the spectrum, only mixed
more or less with white light. We should seek in vain
among them for good representatives of olive-greens or
chocolate-browns and many other common tints. One of
the problems that present themselves most frequently is to
ascertain the complementary colour of some particular pig-
ment or mixture of pigments. For the rough solution of
such questions a method given by Dove can be employed :
A small square of paper, an inch or less in size, is to be
painted with the pigment in question and placed on a sheet

166 MODERN CHKOMATICS.

of black paper, and viewed througli an achromatized prism
of calc spar. This is shown in Fig. 68, and has the property

Fig. 68.— Achromatic Prism of Calc Spar.

of furnishing when held before the eye two equally bright
images of objects viewed through it. It is used in this ex-
periment instead of a plain calc-spar prism, because it gives
a greater separation of the two images, and thus allows the
employment of larger squares of coloured paper. As the
finding of the colour which is complementary to any given
one depends entirely on experiment, a second piece of pa-
per, also an inch square, is now to be painted with the col-
our which it is supposed will be complementary to the
first, and the two painted papers are to be combined together
with the aid of the calc-spar prism. Let us suppose that we
wish to obtain the complement of a dull reddish-brown.
The red-brown square is placed on the black paper ; beside
it we lay a piece painted with a dull bluish-green grey, and
arrange matters so that an image of the red-brown paper
falls on one furnished by the blue-grey paper. If the two
colours are complementary, their joint image will be white,
or rather pure grey. If, instead of pure grey, it shows a
tendency to reddish-grey or bluish-grey, the colour of the
second slip of paper must be modified accordingly. This
operation is facilitated by constantly comparing the tint
obtained with that of a slip of pure grey paper, placed on
the same sheet of black paper. When the process is fin-
ished, the appearance will be that indicated in Fig. 69.

The practical objections to this mode of experimenting
are, that the calc spar reduces the luminosities of the col-
oured papers, and that, owing to the imperfect means of

COMPLEMENTART COLOURS. 167

comparison, one is too apt to accept as pure grey any
approximation to this tint. For all accurate work it is far
better to employ Maxwell's disks in the manner now to be
described.* Let us suppose that we wish to obtain the

Fig. 69. — Eed-brown and Blae-Krey are combined in the central Image, and form a Pnre
Grey. Below is a Grey placed for comparison.

complement to a somewhat dark vermilion-red. The de-
tails in an actual experiment were as follows : A disk was
painted with the tint in question, and combined with two
others painted with emerald-green and ultramarine-blue, as
it was known beforehand that the desired colour would be
a bluish-green of some kind. See Fig. 70, which shows
also smaller black and white disks placed on the same axis
for the purpose of obtaining a pure grey for comparison.
It will be noticed that the red colour has been made to oc-
cupy just one half of the disk, or 50 parts ; the remaining
50 parts are to be divided up between the blue and green,
as is found by experiment necessary. The result showed

* For an account of Maxwell's disks, see previous chapter.. .
8

168

MODERN CHROMATICS.

that 50 parts of red were neutralized by. 31 parts of emer-
ald-green and 19 parts of artificial ultramarine-blue ; the
three colours gave a grey identical with that furnished by
13 parts white and 87 black. Putting this in the form of
an equation, we have : 50 red + 31 em. -green + 19 ult.-blue
= 13 white + 87 black.

Fie. 70.— Emerald-green and TTl-
tramarine-blue Disks arranged
so as to neutralize Ued and pro-
duce wi til it a Pure Grey. Cen-
tral Black and White Disk for
tlie production of a Pure Grey.

Tig. 71.— Disks of Emerald-green
and Ultramarine-blue arranged
so as to give a Colour Comple-
mentary to Eod.

The next operation is to mix emerald-green and ultra-
marine-blue in the proportion of 31 to 19, which will evi-
dently give us the correct complement of our red. As
these two colours in the last experiment occupied exactly one
half of the disk, it evidently will be necessary to double
them if they are to be spread over a whole disk ; accord-
ingly, we combine them together, taking 62 parts of the
green and 38 of the blue. (See Fig. 71.) This compound
disk when set in rapid rotation gives us accurately the com-
plementary colour of our red. It is seldom in practice that
so complete a result as this can be obtained ; for it is evi-
dent that, if the red colour had been more luminous, it
would have been impossible to balance 50 parts of it with
50 parts of the blue and green, however arranged ; the
resultant tint would always have been a reddish-gvej.
Conversely, if the red had been less luminous, a similar

COMPLEMBNTABY COLOTJRS. 169

difficulty would have occurred : the resultant tint would
have always been somewhat bluish green instead of pure
grey.

We give now an actual experiment which illustrates the
way in which the matter usually falls out, and shows at the
same time the nature of the result to be expected. It was
desired to obtain the complement to a dull yellow, some-
what like the tint of brown pasteboard. A disk was painted
with this colour and combined with one of artificial ultra-
marine-blue, the small black and white disks of course
being present. When this arrangement was set in rotation,
it was found impossible to produce a pure grey, however
the proportions of the blue and yellow disks were varied ;
at the best the tint furnished was a purplish-grey. This of
course indicated the necessity of adding some green to the
blue : a disk of emerald-green was now added, when it was
found that 43 parts yellow, combined with 43 parts ultra-
marine-blue and 14 parts emerald-green, gave a grey identi-
cal with that furnished by 24 parts of white and 76 black.
The equation then reads : 41 yellow -j- 45 blue + 14 green
= 24 white + 'J'6 black. Prom this it follows that, by mix-
ing ultramarine-blue and emerald-green in the proportion
of 45 to 14, a colour complementary to our yellow could be
obtained. We then divide up 100 in this ratio, and assign
76'3 parts to the blue and 23*7 to the green, combine the
blue and green disks in this ratio, and by rotation obtain
the complementary colour, which is a fine blue. This blue
is, however, somewhat davker than the true complement of
our yellow, for in the first experiment the yellow did not
occupy fully one half of the disk, or 50 parts, but only 41
parts ; if we had made it fill half the disk, the other col-
ours would not have been luminous enough to balance it,
and grey would not have been produced. It is easy for us
to calculate how much too dark the tint is which we have
obtained as the complement of the yellow : if we call the
luminosity of the true complement 100, then that which we

170 MODERN CHROMATICS.

actually obtain is 69-5.* Hence, in using this complement,
we must always allow for the fact that it is less luminous
than the true complement in the degree above indicated.
Furthermore, no better result could be obtained with the
disks of blue and green which were used ; to improve the
result, it would have been necessary to alter them so that
they would become able to reflect more blue and green light
to the eye. On the other hand, if they had originally re-
flected too much green and blue light, this might have been
diminished with the aid of a black disk, and the true com-
plement accurately obtained. Hence it follows that we can
obtain the true complement to a given colour with accu-
racy only in those cases where we have at our disposal rep-
resentatives of this complementary colour which are sufii-
ciently intense — that is, at the same time luminous and
saturated. The practical effect of this is, that we can not
directly obtain the complementary tints of the most intense
of the warmer pigments, such as carmine, vermilion, red
lead, chrome-yeUow ; the colder pigments, like emerald-
green, cobalt-blue, Prussian-blue, ultramarine, etc., aU fall-
ing considerably below them in intensity.

For many purposes it is convenient to possess a set of
disks arranged in pairs and representing the main comple-
mentary colours. It would of course require much time
and patience to construct a set in which the colours were
quite correct in the matter of hue and also in that of lumi-
nosity ; and in such a set, with the pigments at our disposal,
the red and orange hues would be quite dull, and the yel-
lows little more than browns or olive-greens, for the reason
above given. The author recently constructed a set in
which the hues were nearly correct, and the luminosities as
favourable as could be obtained without too much expendi-
ture of time and trouble. Their relative intensities as
pairs were also determined, as well as the amounts of white
light which they furnished when combined in pairs.

* See appendix to this chapter.

COMPLEMENTARY COLOURS.

171

Table or Complementabt Disks.

Colour.

Intensity.

Colour.

Intensity.

Amount of white
light furnished
by the combi-
nation.

Carmine

Vermilipn

Orange

Yellow

100

100
89-1
S8-1

100

Blue-green. . . .
Green-blue . . .
Greenish-blue .

Blue

French-blue . .

Violet

Purple

68-6
66-2
88-7
64-5

100

100
86-9

25
25-3
2'7-2
25-6

Greenish-yellow. .
GremisJi-jeWovr. .
Green

28-9
32-2
25-1

It will be noticed that an attempt was made to have the
.set so arranged as to furnish in each case, as far as possible,
about the same amount of white light, so that in this re-
spect the disks should have nearly the same rank. For an
account of the pigments that were used, the reader is re-
ferred to the appendix to the present chapter. With the
aid of this set of complementary disks, hundreds or even
thousands of pairs of complementary colours can be quickly
produced. This is effected by combining any pair either
with white or with black, or with both. For instance, all
the reds darker than carmine can be obtained by combining
the carmine disk with different proportions of a black disk,
the corresponding complementary colours being furnished
by the blue-green disk similarly treated ; in the same way,
the reds paler than carmine and their complements are to
be obtained by the addition of a white disk ; and finally,
all the complementary red and blue-green greys are yielded
by adding a white and a black disk to either of the two
coloured disks. Hence it will be seen that, though a set of
disks of this kind costs some trouble at the start, yet after-
ward it more than repays the labour, by quickly supplying
us with a vast range of colours which are either truly com-

172

MODERN CHROMATICS.

plementary in all respects, or defective only in tlie matter
of luminosity to a calculable extent.

We come now to a matter which at first sight will seem
strange. We have seen that every colour has its comple-
mentary colour, but more than this is true : every colour
has many different complementary colours. This may best
be illustrated by an experiment. Let us suppose we wish
to study the colours which are complementary to that of
our green-blue disk : We combine this disk with one of
vermilion, to which it is complementary, so that we have
50 parts of green-blue and as much vermilion as is found
necessary. 'Now, as considerably less than 50 parts of ver-
milion will represent the complement of our green-blue, we
fill up the blank space left by the vermilion with black.
After being adjusted so as to give a grey, the disk was
found to be arranged as radicated in Fig. 72. It was found,
namely, that 50 parts of green-blue were just balanced and
neutralized by 27 parts of vermilion, leaving 23 parts of
the disk to be occupied by black. To render visible this

Fig. 72.— Green-blue and the cotn-
plementary amount of Vermilion.

\ VEBMIUON /

Fig. 78.— This Disk by rotation
gives one of the Complements
of Green-blue.

complement of the green-blue, we combine a black and ver-
milion disk in the proportion 23 to 27, or, what is the same
thing, in that of 46 to 54, and rotate it. (See Fig. 73.) This
furnishes us with a somewhat dark vermilion-red ; it is one of
the complements of the green-blue. If we now replace the

COMPLBMENTAEY COLOUKS. 173

black in Fig. 72 by white, the proportions still remaining
as 46 to 54, we produce a light-reddish flesh tint, quite dif-
ferent ia appearance from the dark-red colour before ob-
tained, but still accurately complementary to our green-
blue, since it contains the same amount of red. If the 46
parts of black are gradually replaced by white, a series of
tints will be obtained differing in luminosity, but all red-
dish, and all complementary to the same green-blue. The
set of complementary disks above described furnishes great
facilities for studying the different appearances assumed by
pairs of complementary colours under these circumstances.

Another point now deserves attention. Suppose that
we select by daylight two painted surfaces with colours
that are strictly complementary — for instance, red and
green-blue. Afterward, if we view these two surfaces by
lamp-light or gas-light, it will not at all follow that the
colours will still neutralize each other and remain comple-
mentary. It is easy to experiment on this matter with the
aid of our set of complementary disks. By daylight it was
found that 41 parts of carmine neutralized 59 parts of green-
blue and gave a true grey : by gas-light these colours were
no longer complementary, but, in the above-mentioned pro-
portions, furnished a pretty strong red-purple. Experiment-
ing still by gas-light, the red was reduced to 29 parts and
the green-blue increased to 71, when the tint of the mixture
became less red, but still neutralization could not be effect-
ed : the two colours had by gas-light ceased to be comple-
mentary, and it was found necessary to add 13-5 parts of
green to reestablish this relation between them. The same
fact was observed with the following pairs of complemen-
tary colours :

Vermilion and green-blue.
Orange " cyan-blue.
Yellow " blue.

174

MODERN CHROMATICS.

With the pair greenish-yellow and ultramarine-blue, the,
efEect -was reversed : it was necessary by gas-light to re-
duce the greenish-yellow somewhat and to replace a portion
of it .by orange. The pairs greenish-yellow and violeti
green and pm-ple, remained complementary alike by day-
light and gas-light. The following table shows the results
when the disks were arranged so as to appear complemen-
tary by daylight and by gas-light :

Coloura.

By dayUght.

By gas-light.

40''? carmine and 59'3 green-blue

36 vermilion and 64 green-blue

47 orange and 53 cyan-blue

Grey.

((

11
a
11

Red-purple.
Purplish-red.
Purplish-red.
Greyish-purple,

Greenish-grey.

Grey.f

39-2 yellow and 60-8 blue

52-'? greenish-yellow and 47'3 Prenoh-
blue*. . ..;

53 greenish-yellow and 47 violet

46'5 green and 53 'o purple

Below follow the proportions when the disks appeared
coraplementary by gas-light :

Colours.

29 carmine, 57 green-blue, and 14 green.. . .
27 vermilion, 57 green-blue, and 16 green . .

37 orange, 50 cyan-blue, and 13 green

37'5 yellow, 56 blue, and 6'5 green

45 greenish-yellow, 48 French-blue, 7 orange. Purplish.

52 greenish-yeWow, 48 violet ' Grey.

47 green, 63 purple "

By daylight

By gas-light.

Strong green.

Strong green-grey
Greenish-grey.

Grey.f

These changes depend on two causes. First, the composi-

* Artificial ultramarine-blue.

f Really a dark yellow, which appeared by gas-light grey.

COMPLEMENTARY COLOURS. 175

tion of gas-light is different from that of white light ; the
violet, blue, and greenish-blue rays in gas-light are com-
paratively feeble, and, owing to this circumstance, the disks
must of course present a different appearance when illumi-
nated with it, the violet, blue, and green-blue pigments ap-
pearing relatively darker. This circumstance would, how-
ever, merely require us to use more of these hues, and would
not necessitate the introduction of foreign colours. The
second cause is that by gas-light we are able to effect neu-
tralization only when the mijcture of the two colours has a
tint similar to that of the general illumination itself, which
in this case is not white, but yellow, inclining toward or-
ange. It follows from these experiments that if red or
orange is to be contrasted with its complement by gas-light,
it will be necessary to make the contrasting colour more
greenish than would be allowable by daylight ; the same is
true to a less extent of orange-yellow and of yellow itself.

Leaving these practical matters for a moment, let us
turn our attention to a couple of theoretical points which
are not without interest. In a previous chapter we have
seen that colour varies with the length of the waves of
light : knowing this, we are very naturally led to inquire
whether there is any fixed relation between the lengths of
waves which produce upon us the sensations of comple-
mentary colours. Upon . studying the matter with the help
of a chart of the normal spectrum, we find that no such
relation exists, owing to the circumstance that the change
in colour in dijQEerent parts of the spectrum is not directly
proportional to the change in wave-length, as was pointed
out in a previous chapter. Helmholtz found that the rela-
tion which does exist is not a fixed one for all the different
pairs of complementary colours, but that it varies consider-
ably. With some of the pairs this relation is as 1 to 1'2,
in -others as 1 is to 1"333 ; or, using the musical notation,
we Would say that the relation varies from that existing
between a note and its fourth to that between a note and

176 MODERN CHROMATICS.

its diminislied third. THs is one of the many facts which
are fatal to any chromatic theory that has a musical basis
for its foundation.

The other matter demanding our attention is the mode
in which the phenomena of complementary colours are ex-
plained by the theory of Thomas Young. In a previous
chapter we saw that a mixture of red, green, and violet
light, when presented to the eye, produced the sensation of
white ; and in the present chapter we have found that this
same sensation can be produced by the mixture merely of
two properly selected colours. Now, according to Young's
theory, the sensation of white is produced when the thi-ee
sets of nerve-fibrils with which the retina is provided are
stimulated to about the same degree of activity ; hence it
must follow that two colours can stimulate all the three sets
of nerves as effectually as the three fundamental colours.
It is this fact that we are called on to account for, and the
explanation in the principal cases is as follows :

Red and green-blue are complementary colours, because
red light stimulates the red nerves, and green-blue light
both the green and violet nerves ; the joint action of the
three sets gives white light. Oi^sa^e and cyan-blue is the
next pair : orange light sets in action the red nerves power-
fully, also somewhat the green nerves ; cyan-blue sets in
action the green and the violet nerves ; all thi-ee sets of
nerves acting, the result is the sensation of white. The
case is much the same with yellow and genuine ultramarine-
blue : both colours stimulate two sets of nerves ; that is,
the yellow acts on the red and green nerves, the blue on
the green and violet nerves. With green and purple the
first colour acts of course on its own set of nerves, the sec-
ond on the red and violet nerves. All this is strictly in
accordance with the principles of Young's theory, as will
be found by reference to the chapter in which it is treated.

This explanation enables us to understand a fact which
otherwise might appear quite strange, viz. : that if we take

COMPLEMENTARY COLOURS. I77

away from white light any colour, the light which remains
will have the complementary hue. Thus, if we strike out
from white light the orange rays, the remainder wiU appear
of a rather pale cyan-blue. The table of complementary
colours explains this result ; thus,

Red and green-blue make White.

Orange and cyan-blue make White.

Yellow and blue make ' White.

Green-yellow and violet make White.

Green and purple make White.

All these five pairs of colours are present in white light.
If we remove from it orange, tljen cyan-blue is the only
colour which is not neutralized ; all the other colours bal-
ance up and make white light, which mixes with and pales
the uncombined cyan-blue. The explanation is the same in
all the other cases. It follows from this that the comple-
mentary colours produced by the method of striking out a
colour are rendered rather pale by the presence of a consid-
erable amount of white light. This is the reason why the
complementary colours obtained by the use of polarized
light are always rather pale. The presence of this white
light, as will be shown in the following chapter, actually
somewhat alters the tint of the coloured light mixed with
it ; red is made to incline to purple, orange to red, purple
and ultramarine to violet.

Before closing this chapter it may be well to make some
remarks concerning the complement of pure yellow, and
the complements of the several varieties of the more com-
mon blue pigments. In different works the complement of
yellow is given as indigo-blue, ultramarine-blue, or simply
as blue. Genuine ultramarine-blue is complementary to
pure yellow, the complement of artificial ultramarine-blue
being a decidedly greenish-yellow. Gamboge gives a yel-
low which is slightly orange ; its complement is the pig-

178 MODERN CHKOMATICS.

ment known as cobalt-blue. The complement of Prussian-
blue was determined and found by the author to be a
somewhat orange-yellow ; it was made by mixing with a
rotating disk 65 parts of pale chrome-yellow with 35 parts
of vermilion. The complement of indigo, used as a water-
colour pigment, was also determined with care, and found
to correspond quite closely with that of Prussian-blue'; it
hence follows that indigo may be considered to be the same
as darkened Prussian-blue, and not to represent, as some
authors have suggested, darkened ultramarine-blue. The
term indigo was applied by Sir Isaac Newton to designate
the more refrangible blue of the spectrum ; to this it does
not really at all correspond in any respect, and in the pres-
ent work the term ultramarine-blue is substituted for it. If
the different blues be arranged in the order of the spectrum,
we shall have cyan-blue, indigo or Prussian-blue, cobalt-
blue, genuine ultramarine and artificial ultramarine, the last
being a violet-blue. Among the yellow pigments the some-
what orange-tiated chrome-yellow is complementary to in-
digo and Prussis(n-blue ; chrome-yellow with a still more
orange hue has a complement nearer to cyan-blue.

APPENDIX TO CHAPTER XI.

The mode of calculating the relative intensities of pigments
which are complemeutaxy'is quite simple, and is here illustrated by
an example. Let us suppose that 25 parts of a certain red neutral-
ize 75 parts of a green-blue. The compound disk will then appear
as in Fig. 74. It is evident that the intensity of the green-blue is
only one third of that of the red, since it takes three times as much
green-blue as red to effect neutralization. Let I be the greater in-
tensity and I' the lesser ; then we have —

25 I = 75 r
Making the greater intensity 100, we have —

APPENDIX TO CHAPTER XI.

25 X 100 = 75 r
I' = 33-3

1T9

Fig. 74.— Disk with 25 Parts Eed and T5 Parts Green-blue.

That is, if we call the intensity of our red 100, that of the green-
hlne will be only 33-3. In the case given in the present chapter we
have —

41 1 = 59 1'

41 X 100 = 59 r

r = 69-5

PIGMENTS USED IN THE SET OF OOMPLEMENTAEY DISKS.

Carmine as a water-colonr ; for its complenjentary green-blue, a
mixture of cobalt-blue and emerald-green.

Vermilion as a water-colour ; for its complement the same as
above, the proportions being changed.

For the first two pairs, then, we can employ two of our most
intense and saturated pigments ; this, however, is not possible with
orange and yellow, without producing disks of a rank different from
the preceding, or obtaining disks which show greater differences in
luminosity than any which have been tolerated in the table given in
the present chapter. Thus a fine orange colour was mixed from
red lead and Indian-yellow, which would have been considered by
most painters, as I suppose, a fair companion for -the carmine and
vermilion; or, if objection had been made, it would have been
rather to its want of intensity. Placing the intensity of this orange
as 100, the intensity of its complement (made of cobalt-blue and
emerald-green) was only 47, a figure smaller than any in the table.

180 MODERN CHKOMATICS.

These two colours, however, furnished a white such as could be
obtained by mixing, with the aid of disks, 36 parts of white with 64
of Mack ; this n amber is considerably higher than those allowed in
the table. The combination then was rejected, because it was faulty
in two respects, and a dull-looking orange substituted for it. This
dull, rather poor-looking orange balanced its complementary cyan-
blue well, and witb it gave 27 per cent, of white light, wMoh was
fuUy up, to the average, and proved that in the matter of luminosity
it belonged in the set rather than the disk just mentioned.

A similar experience was encountered with yellow. Two beau-
tiful disks were prepared with gamboge and cobalt-blue. Setting
the intensity of the gamboge as 100, that/of the cobalt was 90,
which was nearly what was wanted. The combination, however,
gave on rotation a white which was about 100 per cent, too bright,
showing that the two disks belonged in a set sucb as would be fur-
nished by pigments twice as bright as those employed by me ; but
no such pigments exist. This is only another illustration of the
fact, already several times mentioned, that our bright-yeUow pig-
ments, such as gamboge, chrome-yeUow, cadmium-yellow, etc., can
not properly be reckoned as the equal companions of the other pig-
ments ordinarily found on the painter's palette. This circumstance
affects our judgment, and we are surprised at the lack of brilliancy
of the yellow space even in the prismatic spectrum,- and at the fact
that mixtures of red and green light produce yellow light of so infe-
rior a character. On the other hand, the possession of such excep-
tional pigments as the bright yellows and orange-yellows enables
the artist at will to extend his scale of brilliancy in an upward
direction much farther than otherwise would be possible.

The greenish-yellows were made with gamboge mixed with a
little Prussian-blue, the pigments being laid, not on drawing-paper,
but on rather absorbent cardboard, which dulled the colours to a
desirable extent. For violet, " Hoffmann's violet B. B." was em-
ployed, none of the violet pigments used by artists being of the
slightest use on account of their very dull appearance and poverty
in the matter of violet light. The green was made by mixing a
little chrome-yellow with emerald-green ; the purple was " Hoff-
mann's violet R. E. R."

CHAPTER XII.

ON THE EFFECT PRODUOED ON COLOUR BY A CHANGE
IN LUMINOSITY, AND BY MIXING IT WITH WHITE
LIGHT.

In our study thus far of coloured surfaces it has been
tacitly assumed that theu* action on the eye is a constant
one, and that a red surface, for example, will always appear
red to a healthy eye as long as it remains visible. In point
of fact, however, this is not quite true, for it is found that
coloured surfaces undergo changes of tint when they are
seen under a very bright or very feeble illumination. Ar-
tists are well aware that scarlet cloth under bright sunshine
approaches orange in its tint ; that green becomes more
yellowish ; and that, in general, a bright illumination causes
all colours to tend somewhat toward yellow in their hues.
Helmholtz, Bezold, Rutherfurd, and others have made simi-
lar observations on the pure colours of the prismatic spec-
trum, and have found that even they undergo changes
analogous to those just indicated. The violet of the spec-
trum is very easily affected : when it is feeble (that is,
dark), it approaches purple in its hue ; as it is made strong-
er, the colour changes to blue, and finally to a whitish-grey
with a faint tint of violet-blue. The changes with the
ultramarine-blue of the spectrum follow the same order,
passing first into sky-blue, then into whitish-blue, and final-
ly into white. Green as it is made brighter passes into yel-
lowish-green, and then into whitish-yellow ; for actual con-
version into white it is necessary that the illumination should

182 MODERN CHROMATICS.

be dazzling. Red resists these changes more than the other
colours ; but, if it be made quite bright, it passes into
orange and then into bright yellow.

It is remarkable that these changes take place with the
pure colours of the spectrum ; but the explanation, accord-
ing to the theory of Young and Helmholtz, is not diflScult.
Let us illustrate it by an example, taking the case of green
light, which, as we have seen, acts most powerfully on
what we termed the green nerves, less powerfully on the
red and violet nerves. Now, as long as the intensity of our
green light is small, it acts almost entirely on its own pecu-
liar set of nerves ; but, when the green light is made bright-
er, it begins to set into action also the red and to a lesser
extent the violet nerves ; the result of this is that the sen-
sation of white begins to be mingled with that of green, all
three sets of nerves being now to some extent in action.
As in this process the violet' nerves lag behind, the main
modification of the colour at this stage is due to the action
of the red nerves, which cause it to appear more yellowish ;
hence it changes first to a yellowish-green, then to greenish-
yellow, and finally, if the light is very bright, to a whitish-
yellow. Corresponding to this, when red light is made
very bright, the red and the green nerves are set into ac-
tion, the result being that the colour changes in appearance
from red to yellow. In this case the violet nerves play a
secondary part, and their action merely causes this yellow
to appear somewhat whitish. When pure" violet light is
made quite bright, immediately the green nerves begin to
add their action to that of the violet, and the tint quickly
changes from violet to ultramarine-blue ; the red nerves are
soon also stimulated, and, in connection with the green, fur-
nish the sensation of yellow ; this yellow, mixing with that
of the ultramarine-blue before mentioned, gives as a result-
ant tint a whitish-grey with a faint tint of blue or violet-
blue. The explanation of the changes which the interme-
diate colours of the spectrum undergo is analogous to that

EFFECT ON COLOUR BY CHANGE IN LUMINOSITY. 183

just given. The tendency, in all cases is to the production
of a yellowish- white, or to a white, if the coloured light be
very hright. If its brightness be more moderate, the col-
our will still appear paler and as though mixed with a cer-
tain aniount of yellow. Artists, by taking advantage of
these facts, are able to represent in their paintings scenes
under high degrees of illumination. According to Aubert,
the whitest white paper is only 57 times brighter than the
darkest black paper ; and it is within these narrow limits
that the painter is compelled to execute his design : hence
the necessity of employing illusions like the one just men-
tioned. Many effects in nature are beautiful and striking,
as much on account of their high degree of luminosity as
for any other reason. The artist is not able to transfer to
his canvas the brightness, which in this case is really the
attractive element ; but by the use of pale colours, well
modulated, he suggests a flood of light, and we are delight-
ed, not so much with the pale tints as with the recollections
they call up.

We have just examined the remarkable alterations which
the pure colours of the spectrum undergo when their lumi-
nosity is made very great, and pass now to the changes
which occur when the intensity of coloured light is made
very feeble. Von Bezold has made some interesting obser-
vations of this character on the colours of the spectrum.
With a very bright prismatic spectrum he was able to see
a pure yellow near D and a whitish-blue near F, the other
colours being in their usual positions. When the illumina-
tion was only moderately bright, the yellow space dimin-
ished and became very narrow ; the ultramarine-blue van-
ished, and was replaced by violet. With less illumination,
the orange-yellow space assumed the colour of red lead, and
the yellow vanished, being replaced by a greenish tint ; the
cyan-blue was replaced by green, the blue and ultramarine-
blue by violet. The spectrum at this stage presented scarce-
ly more than the three colours, red, green, and violet. With

184 MODERN CHEOMATICS.

a still lower illumination, the violet vanished, the red be-
came red-hrown, and the green was visible as a pale-green
tint ; then the red-brown disappeared, the green still re-
maining, though very feeble. With still less light, even
this suggestion of colour vanished, and the light appeared
simply grey.

The tendency in these experiments is evidently just the
reverse of what was observed where the illumination was
very bright. In that case the coloured light as it increased
in brightness gradually set all three sets of nerves into ac-
tion, and the result was white or yellowish- white ; but here
the action of the coloured light as it grows feebler is more
and more confined to a single set of nerves. From this it
results that those colour-sensations which are due to the
joint action of two sets of nerves speedily diminish when
the colour is darkened, and are replaced by the primary
sensations, red, green, or violet. The sensation of orange
is produced by those light-waves in the spectrum which
have a length such as to enable them to stimulate the red
nerves strongly and the green nerves to a lesser degree ;
hence, when orange-coloured light is made very weak, it
fails to act on the green nerves while still feebly stimulat-
ing the red, and consequently the sensation of orange passes
over into red. For similar reasons the sensations of yellow
and greenish-yellow pass into green, as do also those of
greenish-blue and cyan-blue ; in the same way the sensa-
tions of blue, ultramarine-blue, and violet-blue pass into
violet. It is quite evident that these changes furnish an-
other argument in favour of Young's theory of colour, and
also tend to approve the selection of red, green, and violet
as the fundamental colour-sensations, i

In the experiment of Von Bezold just mentioned, after
the spectrum had been darkened to a certain degree only
three colours remained — red, green, and violet ; this dark
red, however, as far as sensation goes, is somewhat changed
in character, and, according to the unpublished experiments

EFFECT OX COLOUK BY CHANGE IN LUMINOSITY. 185

of Charles Pierce, has become somewhat purplish ; the same
is true of the green, which is more bluish ; the violet alone
is unchanged. Now, just these same effects can be produced
by mixing small quantities of violet with red or green ;
hence the final effect of darkening on all the colours of the
spectrum is virtually to mix them with increasing quantities
of violet light. The cause of these peculiar changes, ac-
cording to the theory of Young and Helmholtz, resides in
the fact that the violet nerves act more powerfully, rela-
tively to the red and green nerves, when the light is feeble.
For example, if we present to the eye pure green light, it
will stimulate the green nerves strongly, the red and violet
to a much less degree : we thus obtain a certain sensation,
and call it green. If now we greatly diminish the intensity
of the green light, it will of course affect the green nerves
to a minor degree ; but, besides this, it has now less action
on the red than on the violet nerves, so that virtually we
have a mixture of green and violet, which will cause the
green to appear bluish-green. The same explanation holds
good for the red, dull red light producing less effect on the
green than on the violet nerves.

The change which colour undergoes when darkened is
interesting from a practical point of view ; and accordingly
the author made a series of experiments on this subject,
using for that purpose coloured disks and the method of
rotation. In these experiments we do not deal with the
pure colours of the spectrum, but with surfaces painted
with brilliant pigments, which correspond more nearly to
the cases that present themselves to the artist and decora-
tor. A black disk was in each case combined with a col-
oured disk, as indicated in Fig. 75 ; a smaller disk of the
same colour being either attached to the axis for compari-
son or held from time to time near the rotating disk. It
was ascertained by previous experiments that the amount
of white light reflected by the black disk was small ; if we
set the amount of light reflected by white cardboard as 100,

186 MODERN CHROMATICS.

then the black disk which was employed on this occasion
reflected two per cent, of white light, or -^. The colour of
the painted disks was in every case as intense, saturated.

Fig. To.— Chrome-yellow and Black Disks Fig. T6.— The disk of Fig. 75 when in
in combination. rotation becomes coloured olive-green.

and brilliant as possible. The results obtained by rotation
— that is, by reducing the luminosity of the colours by mix-
ing black with them — are briefly indicated below :

Table I.

Name of Colour. Effect of reducing its Laminosity.

Fuodamental red (carmine ) „ , , , ,

J ... , > Not changed, or made shghtly purplisQ.

Vermilion More red, less orange-red.

Red lead More red, less orange-red.

Orange Brown.

Chrome-yellow or gamboge. Olive-green.

Greenish-yellow More greenish.

Yellowish-green More pure green.

Fundamental green Not changed, ormades%M!/ more bluish.

Emerald-green More green, less blue-green.

Blue-green More green, less bluish.

Cyan-blue More greenish.

Prussian-blue Dark grey-blue (not changed).

Cobalt-blue Dark grey-blue (not changed).

Ultramarine-blue (artificial). More violet, less blue.

Violet Dark violet.

Purple More violet, less red.

Carmine Not much changed.

EFFECT ON COLOUR BY CHANGE IN LtTMINOSITT. 187

It will be noticed that these results correspond more or less
closely with those of Von Bezold, before given.

Some of the changes in the experiments just mentioned
were so great as to be quite astonishing, and might well
tempt the beholder to believe that the black disk exercised
some peculiar influence on the result ; this, however, was
not the case, as the same results can be obtained without
the black disk by simply reducing the illumination of the
coloured disks by holding before the eye two Nicol's prisms,
and turning them so as gradually to cut off the coloured
light. On the other hand, if the tints that are obtained by
using the black disk give the true appearances of surfaces
painted with pure pigments, but viewed under a feeble il-
lumination, then accurate copies of them ought, when power-
fully illuminated, to appear once more brightly coloured,
and of the original tints. This was found to be the case,
for example, with gamboge, whei-e the change in colour by
darkening was from a slightly orange-yellow to a fine olive-
green. The olive-green colour was carefully copied with
water-colours on a slip of paper, and afterward held in
bright sunlight ; this caused it to appear yeUow, and made
its colour resemble that of the gamboge disk placed near it,
but in the shade.

The general result of these experiments is, that, if the
illumination is feeble, the colours become weaker, and there
is on the whole a general tendency toward a darkish blue ;
just as in the reverse case, where the colours are made very
bright, there is a tendency toward a whitish-yellow. This
average tint can best be studied by observing moonlight
effects : here the more luminous colour appears to be a
somewhat greenish-blue, the darker shades more like an ultra-
marine-blue. With regard to this delicate point, the paint-
ers of moonlight landscapes are as good an authority as we
have, and the best of them are very decided as to the prev-
alence of various shades of blue, greenish-blue, and violet-
blue. Similar effects, though smaller in degree, are ob-

188 MODERN CHROMATICS.

served on dull, cloudy days, when the prevailing tint is a
bluish-grey. Indeed, as Helmholtz remarks, simply view-
ing a sunlit landscape through a pale-hlue glass suggests the
idea of a cloudy day ; while reversing the process, and
viewing a landscape on a dull, cloudy day through a pale-
yellow glass, gives the impression of sunshine. Correspond-
ing to thisj accidental streaks of yellow ochre or sawdust
on the shaded pavement often suggest forcibly the idea of
stray sunbeams ; and other examples of this kind of illu-
sion might be mentioned. If we mix lampblack directly
with pigments on the palette, their colour will of course be
darkened, but the effects produced are not identical with
those obtained by the method of rotation. Paper was
painted with a strong wash composed of carmine and lamp-
black, which imparted to it a dark-reddish, piu-plish hue.
From this a disk was cut and an attempt made to match its
tint by mixing, according to the method of rotation, car-
mine and lampblack. In order to accomplish this, it was

Fig. 7T.— Small central Disk composed of CardLoard washed "with a Mixture of Car-
mine and Lampblack. This is nearly matched by disks painted with carmine, black,
and white, in the proportions indicated.

found necessary to introduce into this rotation-mixture a
quantity of white ; the best match being effected when the
compound disk was arranged as indicated in Pig. 77. This
shows that the saturation or intensity of a coloured piigment
is greatly reduced by mixing lampblack with it on the pal-
ette, and is one reason why artists refuse to adopt this

EFFECT ON COLOUR BY CHANGE IN LUMINOSITY. 189

method of producing dark shades of colour. The mechani-
cal mixture of lampblack with pigments, besides reducing
their saturation, also usually at the same time changes some-
what their hue. In the experiment just mentioned, after
matching the two colours as well as possible, it was found
that the carmine which had been mixed with lampblack on
the palette was more violet in hue than that which had been
mixed with black optically. Corresponding results were
obtained with vermilion when mixed mechanically and op-
tically with lampblack. In the first case the colour was
more of an orange-red hue than in the last. Prussian-blue
and lampblack on the palette give a much more greenish
tint than when mixed by rotation, and similar changes can
be observed with many other pigments.

We have seen thus far that, as we change the luminosity
of a coloured surface, so do we at the same time affect its
hue, all coloured surfaces when very bright tending toward
a whitish-yellow tint. Changes in luminosity, however,
produce still other effects which are quite remarkable. If
we arrange by ordinary daylight sheets of red and blue paper,
which have as far as we can judge about the same degree
of luminosity, and then carry them into a darkened room,
we shall be surprised to find that the blue papers appear
brighter than the red. Indeed, the room may be dark-
ened so as to cause the red paper to appear black, while
the blue still plainly retains its colour. These facts seem
first to have been recorded by Purkinje and Dove. By
similar experiments it can be proved that red, yellow,
and orange-coloured surfaces are relatively more luminous
when exposed to a bright light than blue and violet sur-
faces ; the latter, on the other hand, have the advantage
when the illumination is feeble. Thus Dove noticed a long
time ago that this circumstance disturbs somewhat the bal-
ance of the colours in paintings, if the observer lingers in
a picture gallery as the twilight deepens. From this it
follows that the chromatic composition of a painting should

190 MODERN CHROMATICS.

be somewhat varied, according as tlie picture is likely gen-
erally to be seen under full daylight or in a darkened room.
More attention would no doubt be paid by artists to this
point if they were not obliged to contend with a still more
serious obstacle in the large change which the tint of the
illuminating light undergoes, according as daylight or gas-
light is employed.

It follows from what has just been said that photomet-
ric comparisons of the brightness of differently coloured
surfaces, if made under bright daylight, will no longer hold
good in twilight, and that consequently we can. not under a
certain illumination establish photometric relations that
shall hold good under all other illuminations. For exam-
ple, we may find under ordinary daylight that a certain
piece of blue paper is just half as luniinous as a piece of
red paper ; but it by no means follows that this statement
will be true in a darkened room. Helmhbltz found that
even the pure colours of the spectrum act in this same man-
ner, particularly yellow and ultramarine-blue, or greenish-
yellow and violet, the changes with the other colours
being smaller. This fact suggests an interesting experi-
ment : Yellow and ultramarine-blue are complementary,
that is, together make up white light ; suppose now we
mingle yellow and ultramarine-blue so as to produce white,
a high degree of illumination being employed ; will they
still produce white if they are both correspondingly dark-
ened? We might very naturally suppose that the blue
would not outweigh the yellow, and that instead of white
we should obtain bluish-white. This was, however, found
by Helmholtz, using pure spectral colours, not to be the
case : the darkened mixture of yellow and blue stUl exactly
resembled sunlight which was correspondingly darkened.
The same result was also obtained when a mixture of an-
other pair of complementary colours, greenish-yellow and
violet, was used. These results apparently contradict the
statement that yellow or greenish-yellow acts more power-

EFFECT ON COLOUR BY CHANGE IN LUMINOSITY. 191

fully on the eye when bright than blue or violet. Helm-
holtz accounts for it in this way : Our standard for bright
white is bright sunlight ; we cause the mixture of yellow
and blue to match this sunlight, and then call it white ; we
then darken our sunlight very much, and make it our
standard for a feeble white having a small degree of lumi-
nosity ; we call it darkened white or pure grey, and find
that our darkened mixture of blue and yellow still matches
it perfectly. But, according to Helmholtz, this pure grey
is really somewhat bluish, and it is owing to this circum-
stance that it is able still to match the darkened yellow and
blue, which is also really bluish. Pure grey has always ap-
peared to the present writer as somewhat bluish compared
with pure white, and the following experiment tends to
show that this is indeed the case : A pure grey was gen-
erated on a rotating disk by mixing fifty parts of white

Fig. 78.— Small White-and-BIack Disk arranged so as to make a Grey by rotation. This
grey looks more blaiBh than the larger white disk ; n per cent, of an indigo disk la
then mixed with the white.

with a like proportion of black. This compound disk was
placed on the same axis with a white disk, but when set in
rotation the grey portion appeared slightly more bluish
than the white. In order to cause the white disk to match
in hue (not in luminosity) the grey disk, a disk painted
with a tolerably deep wash of indigo was combined with
the white disk, as indicated in Fig. 78. An assistant ar-
ranged the disks and made the measurements, while the

192 MODERN CHROMATICS.

author simply ordered the proportion of blue to be in-
creased or diminished till the result seemed satisfactory ;
at the termination of the experiment he was informed of
the result. The amounts of added blue in nine consecutive
experiments were as follows, in percentages : 17, 20, 18, 16,
13, 14, 21, 19, 16 ; average, 17. According, then, to these
experiments, it was necessary to add to white 17 per cent,
of a strong tint of indigo, in order to cause it to match in
hue a grey disk made up of equal parts of white and black.
It may be remarked in this connection that the addition of
the blue to the white, although slightly changing its tint,
produced no particularly noticeable effect on its luminosity ;
that is, it was only a little darkened..

All these phenomena can be explained by the theory of
Young as modified by Helmholtz. According to this the-
ory, the sensation of white is produced when the red,
green, and violet nerves of the retina are stimulated to
about the same degree of activity ; furthermore, with a fee-
ble degree of stimulation of all three sets of nerves, the
activity of the violet nerves predominates over that of the
green, and that of the green again over that of the red.
"When the stimulation is made powerful, these conditions
are reversed, the red nerves leading, the green and violet
following. These relations of nerve-action are indicated
by three curves in Fig. 79. The horizontal line represents
increase of actual intensity of white light ; thus the portion
AB stands for feeble white light, A C for white light which
is twice as strong, etc. The vertical line B R measures the
intensity of the red sensation produced by this feeble white
light ; B G and B V give the strength of the sensations in
the case of the green and violet nerves. We see that the
violet sensation, as it is represented by the longest line,
prevails over the others, and that the light, instead of ap-
pearing white, will be such as would be produced by mix-
ing equal parts of. the sensations red, green, and violet ;
i. e., by mixing the sensation of white with a little green

EFFECT ON COLOUR BY CHANGE IN LUMINOSITY. 193

and a little more violet. Now, as we have previously seen
in Chapter IX., the sensations of green and violet when
mixed produce that of blue, so that on the whole we have a
mixture of the sensation of pure white with blue, i. e., blu-
ish-white. If we examine in the same way the line D V G R,
we shall find the conditions reversed ; here we have the
sensation of pure white mixed with a slight excess of that
of green, and again with a little more of red ; as green and

FiQ. 79.— Three Curves showing the action of the Eed, Green, and Violet Nerves when
stimulated by White Light of different degrees of Brightness.

red when mixed furnish yellow, our result now will be
white mixed with yellow, or yellowish-white. This same
diagram also represents in a symbolic manner the fact that
red surfaces are most luminous by bright light, and violet
surfaces by feeble light. It can also be used to explain
the changes which pure colour undergoes when made very
bright or very pale, after the manner employed in the early
part of the present chapter.

We have examined now with some detail the relative
changes in luminosity which coloured surfaces undergo
when exposed to bright and feeble illuminations ; but, be-
fore leaving this part of our subject, it may not be amiss to
mention the fact that all comparisons between the luminosi-
ties of differently coloured surfaces are quite valueless as
the expression of objective facts. An illustration will make

194 MODERN CHROMATICS.

this clear. Suppose we compare together by the eye two
white surfaces or two red surfaces, and find that they ap-
pear to us equally luminous ; now, this will he not only a
fact as far as the eye is concerned, hut it wUl he an objec-
tive fact in nature ; it wiU be equally true in a mechanical
sense, and on, converting our two masses of white light or
red light into heat, we shall obtain equal amounts of heat,
tf, however, we take two differently coloured surfaces, red
and blue for example, and make them equally luminous,
equally powerful so far as the eye is concerned, and then
convert the light they reflect into heat, a delicate thermo- "
metric apparatus will speedily inform us that we are not
dealing in the two cases with equal amounts of force. In
fact, the maximum heating effect was found by Melloni to
be produced by the ultra-red rays which are quite invisible
to the eye ; here, from an objective point of view, resides
the greatest force, but the waves are too long to affect the
eye at all. From this it is evident that the intensity of our
visual sensations depends not only on the strength (height
or amplitude) of the waves of light, but also on then-
length ; the maximum effects being produced by yellow
light, which affects simultaneously the red and green nerves.
In spite of the fact that photometric comparisons of differ-
ently coloured surfaces have no objective value, still for our
purposes they may often be quite precious, or even actually
indispensable, if we propose to give our work a quantitative
character.

Having now considered the changes which colour under-
goes when made very luminous or very feeble, we proceed
to study the effects produced by mingling white with it.
The general result can be expressed quite concisely : the
colour becomes paler, and when the proportion of white is
made large entirely disappears, leaving recognizable only a
white surface. When a disk painted as in Fig. 80 is ro-
tated, the red by mixture with the white gives a ring of
pale red, like that indicated in Fig. 81. Upon reducing the

EFFECT ON COLOUR BY CHANGE IN LUMINOSITY. 195

amount of the colour, it becomes very whitish and pale ;
and Aubert found that the red when mixed with from 120
to 18,0 parts of white entirely disappeared. If we set the
luminosity of vermilion as one fourth of that of white paper,
it follows from Auhert's experiment that mixing vermilion

Fia. 80.— White Disk partially Fig. 81.— Indicates (he sppear-

palnted Eed with Vermilion. ance presented by the previ-

ouB disk when set in rapid
rotation so as to mix the red
and white hght.

with 720 parts of white light, having a brightness equal to
its own, causes the red colour to disappear. Or we may
express the fact thus : Take red light and white light of
equal intensities ; then, if one part of red light be presented
to the eye simultaneously with 720 parts of white light, the
eye will be unable to recognize the presence of the red con-
stituent. Smaller quantities of white light produce very
great changes in the appearance of the colour. _ If we rotate
a disk like that indicated in Fig. ISf, we shall be surprised
to find that, though one quarter of the disk is covered with
vermilion, yet the resultant red tint is quite pale. In this
case twelve parts of white light are mixed with one part of
equally bright red light, and when stated in this manner
the result seems more natural.

When we undertake to study more carefully the mix-
tures of white with coloured light, certain curious anoma-
lies present themselves. If we arrange disks of artificial
ultramarine-blue and white, in the way shown in Fig. 82,

196 MODEKN CHROMATICS.

and set tliem in rapid rotation, we shall find that the addi-
tion of white, instead of producing merely a paler blue,

<^^^OLei>

( ULTRA \
MARim

Fig. 82.— White Cardboard Disk Fio. 83.— Indicates the appear-

partially painted with Ultra- ance produced by mixing white

marine-blue (artificial). with ultramai'ine-blue light.

actually changes the colour to a pale violet. (See Fig. 83.)
If we substitute orange for the ultramarine-blue, the pale-
orange hue generated by rotation shows a tendency toward
red. These two facts have been known for a long time,
and various explanations of them have been proposed.
According to Briicke, ordinary white daylight is itself
slightly reddish in tint, and, when we mix white light with
coloured light, we really add at the same time a little red ;
hence these changes. Aubert, on the other hand, following
a suggestion of Helmholtz, supposes that the true pale shade
of ultramarine-blue is actually violet, but that oui- judg-
ment is perverted by experience drawn from the colour of
the sky, which according to him is a greenish-blue, and re-
tains this tint when mixed with white. This is an explana-
tion which artists would hardly accept, and the experi-
ments given below show that both it and the one previously
cited are insufficient. In an examination of this matter it
was found by the author that changes of this kind are not
confined to the colours orange and artificial ultramarine-
blue, but extend over all the colours except violet and its
complement greenish-yellow ; the main results are given in
the following table :

EFFECT ON COLOUR BY CHANGE IN LUMINOSITY. 197

Table • II. — Showing the Effects op mixing White with Coloured

Light.

Name of Colour. Effect of adding White.

Vermilion. , More purplish.

Orange More red.

Chrome-yellow More orange-yellow.

Pure yellow More orange-yellow.

Greenish-yellow Paler (unchanged).

Green More blue-green.

Emerald-green More blae-green.

Cyan-blue More bluish.

Cobalt-blue A little more violet.

Ultramarine (artificial) More violet.

Violet Unchanged.

Purple. Less red, more violet.

It follows from these experiments that, when we mix white
with coloured light, the effect produced is the same as
though we at the same time mixed with our white light
a small quantity of violet light. Such mixture would ac-
count for all the changes given in the table, as could be
shown by reference to the colour-diagram explained in the
next chapter. This, of course, is only stating the facts in
different language, and is not an explanation.

In the experiments just mentioned the light from bril-
liantly coloured disks was mixed by rotation with from 5
to 50 per cent, of white light, some of the disks requiring a
larger admixture of white light than others to produce the
changes in hue recorded in the table. If now we first
darken our colours very much, and then add a little white
to them, the results will again be somewhat different from
those given in the table, because here two causes are at
work, which sometimes produce opposite results, as we can
see by a comparison of Tables I. and II. In the next set of
experiments, in every case except that of chrome-yellow, 5
parts of the coloured disk were combined with 90 parts of
pure black and 5 parts of white ; 10 parts of chrome-yel-

198 MODERN CHROMATICS.

low were combined with 85 parts of pure black and 5 of
white. The changes of hue are given below :

Table III. — Showing the Effect of making Coloub very Dark and add-
ing TO IT A SMALL PORTION OF White. (The experiments were made by
rotating disks.)

Vermilion became a Dull greyish-purple.

Orange became a Brown (slightly bluish).

Chrome-yellow became a Greyish olive-green.

Emerald-green became a Dark green (less bluish).

Cyan-blue became a Dark greenish-grey.

Prussian-blue became a Dark grey-blue.

Cobalt-blue became a Dark grey-blue.

Ultramarine (artificial) became a Dark grey violet-blue.

Violet became a Dark grey-violet.

Purple became a Dark grey-violet (less red).

In many of these cases the results are similar in charac-
ter to those given in Table 11. This, however, is not the
case with chrome-yellow, as in one case it was made to
appear more orange, while in the other it became a whitish
olive-green. It is evident that in this instance the effect of
darkening the colour overbalanced that of adding white to
it. With emerald-green and cyan-blue a similar result
seems to have been reached, though the phenomena were
less decided. The general effect, then, of jSrst reducing
greatly the luminosity of colour and then adding small
amounts of white, is the production of greys which have a
tendency toward blue or violet, this being the case even
when the original colour is as decided as that of vermilion.
The experiments given in the last two tables will account
for the fact that it is almost impossible to produce a fine
red with the aid of polarized light, the tint being always
rather of a rose-colour, that is, showing a tendency toward
a purplish hue.

It has been shown in the preceding pages that the effect
of mixing white with coloured light is to cause the colour
to become paler, and at the same time to change it slightly,

EFFECT ON COLOUR BY CHANGE IN LUMINOSITY. 199

as though simultaneously a small amount of violet light
had been added to the mixture. This fact naturally sug-
gests an experiment like the following : Suppose we com-
bine a purple and a green disk as indicated in Fig. 84,
employing equal parts, and thus obtain a pure grey. Let

Fift. 84. — Purple and Green Disk ;
when rotated, it makee a Pure
Grey.

Fig. 8S.— Purple, Green, and White
Disk : when rotated, it makes a
Pure Grey.

US now replace 10 parts of purple by white, also 10 parts of
the green by white (Pig. 85) : will we then still obtain a
pure grey, or will the grey be tinged with violet ? Several
experiments of this kind have been accurately made by the
author, but in every case the result was the production of a
grey identical with that given by mixing by I'otation black
and white. The explanation would seem to be that the
green and purple instantly combine to produce the sensa-
tion of grey, and then of course adding white to this grey
can only make it paler, but can not at all alter its tint. It
would seem from this that, when a colour is altered in the
manner above described by admixture with white, time
comes in as a necessary element in the process ; the mixture
of white and coloured light must be allowed to act on the
eye undisturbed during an interval of time which is not too
short, otherwise these peculiar effects will not be produced.

One might suppose that the same result would be pro-
duced by spreading thin washes of coloured pigments on

200 MODERN CHROMATICS.

white paper that is obtained hj mixing white with coloured
pigments by the method of revolving disks. The tint ia
these two cases, however, is usually somewhat different.
A pale wash of carmine, for example, was allowed to dry
on white paper, and an effort was made to imitate it by
combining a deep-coloured carmine disk with one of white
cardboard by the method of rotation. It was soon ascer-
tained that the hue of the water-colour wash was consider-
ably more saturated or intense than a tint of equal luminos-
ity produced by the rotating disks ; it was also found to
be more of a pm'plish hue. When the luminosities in the
two cases were made equal, the water-colour wash showed
far more colour than did the simple mixture of the red and
white light. Treated in the same way, a thin wash of ver-
milion was more orange in hue than a mixture of vermilion-
coloured light with white light ; a thin wash of gamboge
looked yellow, while the mixture by rotation had more of
an orange-yellow appearance. The reason of these changes
is quite evident, and lies in the well-known fact that thin
layers of coloured substances have in general a different
absorptive action on white light from thicker layers of the
same substances. A thin layer of vermilion allows, for ex-
ample, more of the orange rays to pass ; hence in very thin
layers this pigment is sometimes used by artists to represent
very pale tints of orange, or even of orange-yellow. The
other fact above mentioned, viz., that thin layers are often
relatively more saturated than those that are thick, is to be
explained in a different way. It was shown in Chapter X.
that, when a pigment is mixed with one of a different col-
our, not only is the hue changed, but an effect is produced
as though at the same time some black had been added to
the mixture. It now appears that, even when a pigment is
made darker by mixing with it a larger quantity of itself,
a similar change is to some extent produced, the darker
wash of the pure pigment acting as though some black were
mingled with it. In the experiment with the pale wash of

EFFECT ON COLOUR BY CHANGE IN LUMINOSITY. 201

carmine it was actually found necessary to combine black
by rotation with the water-colour wash, so as to reduce it,
before it could be matched by a disk composed of white
and a deep tint of carmine.

The fact now under consideration can perhaps be ren-
dered more intelligible by a different statement. Carmine,
as we know, absorbs powerfully nearly all the coloured rays
of light except the red ; these latter it reflects in considera-
ble quantity, and to this circumstance its red colour is due.
But the experiments just mentioned indicate that it absorbs
also to a considerable extent eyen the red rays, so that a
deep wash of carmine sends to the eye less red light than
we should expect. The author found that yellow glass pre-
sented a parallel case. Yellow glass transmits the orange-
yellow, yellow, and greenish-yellow rays abundantly, and
to this power mainly its yellow colour is due. But it does
not transmit even these rays at all as perfectly as ordinary
window-glass. In one experiment it was found that a plate
of yellow glass absorbed about 25 per cent, even of these
rays. Most coloured substances, pigments, and glasses
probably act in a similar way.

CHAPTER XIII.

ON THE DURATION OF THE IMPRESSION ON THE
RETINA.

Among different forms of fireworks none excite more
admiration than revolving wheels of fire with their brilliant
colom-s, ruhy, emerald, or sapphire, and their wonderfully
blended surfaces, which so often suggest fanciful resem-
blances to roses, carnations, and other flowers. It is quite
possible to arrange matters so as to obtain an instantaneous
view of one of these fiery objects, without at all interfering
with its rapid movement ; and when this isdone, it, is seen
that much of its beauty depends upon an illusion : the
broad, variegated, shaded surface vanishes, and we have
before us simply a few jets of coloured fire, in no wise par-
ticularly remarkable. The appearance of these brilliant
objects depends, then, upon an illusion, and this has for its
foundation the fact that the sensation of sight is always
prolonged after the light producing it has ceased to act on
the eye.

The most familiar illustration of this fact we find in an
old experiment, which no doubt was the parent of our re-
volving fireworks : If a lighted coal on the end of a stick
is caused to revolve rapidly, it describes a ring of fire which
is plainly seen at night to be quite unbroken. The light
from the moving coal falls upon the retina of the eye, and
an image of it is produced, let us say at the point 1, Fig.
86 ; an instant afterward, owing to its having moved into a
new position, the image will be found at 2 and then at 3,

DURATION OF THE IMPKESSION ON THE RETINA. 203

and so on all the way around the circle. Now, if the sen-
sation due to the first image lasts while the circle is being
traversed, then it wUl be renewed before it has a chance to
fade out, and consequently will be present continuously ;
the same will be true of all other points on the circle, which
consequently will produce on the beholder the appearance
of an unbroken ring of light. In order to produce this
condition of things, it is necessary of course that the coal
of fire should move with a certain velocity ; according to

Fio. 86. — Appearance of a Coal of lire in Three Positions.

the observations of D'Arcy, it is essential that it should
traverse its circular path completely in thirteen hundredths
of a second.

It is very easy to experiment on matters like these with
the aid of revolving disks. If we take a circular disk
which is painted black, and has on it a white spot like that
represented in Fig. 87, and set it in rotation, as soon as the
motion is quick enough we shall see a ring of white, just as
in the previous case we obtained a ring of fire. Fig. 88
shows the appearance of the disk when in rapid rotation.

204 MODERN CHROMATICS.

The duration of the sensation of light, or duration of the
impression on the retina, as it is called, varies with the in-
tensity of the light producing it, and in the case of our
white paper is not by any means so great as with the coal
of fire. According to an experiment of Helmholtz, the im-
pression on the retina lasts in this case, with undiminished
strength, about one forty-eighth of a second ; hence it is
necessary for the disk to revolve forty-eight times in a sec-
ond in order to produce the appearance of a steady; uniform
ring of light. While, as just stated, the impression lasts
with undimiaished strength for one forty-eighth of a sec-
ond, its total duration with decreasing strength is much
greater, being perhaps as high as one third of a second,
though this interval varies somewhat with the circum-
stances, and is a little difficult of determination.

Fie. 87.— Disk with White Sector. Fig. 88.— Disk" of Fig. 87 in Eapld

Eotation.

It is not, however, to be supposed that, in the experi-
ment indicated in Fig. 88, the ring of white light will have
the same degree of luminosity as its source, viz., the slip of
white paper pasted on the black disk ; on the contrary, the
luminosity of the ring will be much feebler than that of the
spot. The reason of this is quite evident : we have virtu-
ally spread out the light of the spot over a much larger
surface, and it will be proportionately weaker ; if the sur-
face of the ring is one hundred times as great as that of
the spot, then the luminosity of the ring will be exactly one
hundredth of that of the spot. That this relation is always

DURATION OF THE IMPRESSION ON THE RETINA. 205

strictly quantitative can be proved by a photometric ar-
rangement like that used by Plateau for this purpose, or by
the aid of a crystal of calc spar vrhich divides ordinary light
up into two beams of equal intensity. In this latter case
we arrange a disk by making half of its surface white, the
other half black, as represented in Fig. 89 ; alongside of it,
on a black ground, we place a strip of the same white pa-
. per ; the disk is then set in rotation, and assumes a grey
appearance. With the aid of the cale-spar prism we then
view the strip of paper, which will appear doubled, as shown
in Fig. 90, each image having one half the brightness of the

Fis. 89,— Arrangement ot Disk and
strip for Photometric Comparison.

Fig. 90. — Appearance of Disk In Bots-
tlon, the Strip being viewed through
a Calc-Spar Prism.

original strip ; but either of these images will be as bright
as the grey disk, showing that the luminosity of the grey
disk is just one half of that of the white. This gives an
idea of the method of proceeding, but numerous corrections
must be introduced, some of which will no doubt suggest
themselves to the ingenious reader ; of them we will men-
tion only one, viz. : that, according to the recent investiga-
tions of Wild, the two images furnished by calc spar do not
really have exactly the same luminosity, but differ by about
three per cent.* Dove has proved that the relation we
have been speaking of above also holds good for coloured

* Poggendorff's " Annalen," vol. cxviii., p. 225.

306 MODERN CHROMATICS.

light. This fact is of much importance to us, for on it is
hased the principle involved in Maxwell's disks, which we
have already found so indispensable in quantitative investi-
gations on colour.

The duration of the impression on the retina in the case
of light of different colours has not yet been studied care-
fully. Some experiments were made by Plateau with pa-
pers coloured by gamboge, carmine, and Prussian-blue, and
it was ascertained that in these cases the duration differed
somewhat. Dr. Wolcott Gibbs suggested to the author a
method which would probably solve this problem in a satis-
factory manner, and which is about as f oUows : "With the
aid of a spectroscope a diffraction spectrum is to be pre-
sented to the eye in the form of a series of contiguous col-
oured bands, this division into bands being effected by a
suitable diaphragm placed in the eyepiece of the instru-
ment. In front of the slit of the spectroscope a revolving
disk with one or more openings should allow the light to
enter the instrument ; and, by regulating carefully the ve-
locity of rotation, it would be possible to seize the exact
moment when one or more of the coloured bands ceased to
flicker, and presented a steady, uniform appearance. This
observation would give correctly the interval during which
the impression remained with undiminished strength on the
eye, in the case of the selected colour.

If quite bright light like that from a window is present-
ed to the eye, the impression lasts several seconds — of course
with diminishing strength. The experiment is easily made,
and the observer will find, after the eyes have been closed,
that there is time enough to recognize a good many details
before the disappearance of the image. With intensely
bright light like that of the sun, the image lasts several
minutes, and finally fades out after having undergone a se-
ries of complicated changes in colour. It may perhaps be
as well to mention here a fact which must be borne in mind
•when we undertake to study the after-sensations that follow

DURATION OF THE IMPRESSION ON THE KETINA. 207

the action of -white or coloured light on the eye. If the
light be allowed to act for a short time on the eye, when it
vanishes, as before stated, the sensation remains for a frac-
tion of a second, this after-sensation being in all respects
identical with the original sensation, except that it gradual-
ly becomes weaker and weaker : thus, if the original sensa-
tion is red, the after-sensation will entirely correspond to
this colour. This after-image, which is the only one thus
far treated of in this chapter, is called the positive image.
After a little while, however, the positive image vanishes,
and is replaced by an image of a different character, which
is known as the negative image : thus, if the light original-
ly acting on the eye was red, the negative image is coloured
greenish-blue, or has the complementary colour. These
negative images are quite important, as many matters con-
nected with contrast depend on them, and they wUl be con-
sidered at length in Chapter XV.

We have seen that the positive after-images are useful
in furnishing us, in the case of revolving disks, with a mode
of mixing together masses of coloured light in definite pro-
portions ; and it may be well to mention some other cases
where these images play an important part. The appear-
ances presented by water in motion depend largely on them.
Thus, if we study the ocean waves under direct sunlight, we
shall find that much of their character depends on elongated
streaks of light, which serve to interpret not only the forms
of the larger masses of water, but also the shapes of the
minor wavelets with which these are sculptured. If now
we examine these bright streaks, so well known to artists,
with a slowly revolving disk having one open sector, we
shall find that in point of fact there are no streaks at all
present, but simply round images of the sun, which, owing
to their motion, become thus elongated. Instantaneous
photographs give the same true result, and hence appear
false. An analogous action takes place even in cloudy
weather, and streaks of light are produced which give the

208 MODERN CHROMATICS.

waves a different appearance from -what they -would have if
suddenly made solid, while yet retaining all their glassy
appearance. Again, for the same reason, waves breaking
on a heach appear to us different from their instantaneous
photographs : when viewing the real waves we obtain an
impression which is made up of the different views rapidly
presented during several minute intervals of time, whereas
the photograph gives us only -what takes place during one
of these small intervals. All this applies also more or less
to the case of falling water, as fountains or waterfalls, and
explains the transparency of rapidly revolving wheels.
Owing to the same cause, the limbs of animals in swift
motion are only visible in a periodic way, or at those mo-
ments when their motion is being reversed ; during the rest
of the time they are practically invisible. These moments
of comparative rest are seized by artists for delineation,
whUe the less discriminating photograph is as apt to repro-
duce intermediate positions, and thus produce an effect
which, even if quite faithful, still appears absurd.*

* For a complete list of what has been published on this whole subject,
see a memoir by J. Plateau published by the Belgian Academy of Sciences
in Win.

CHAPTEK XIV.

ON THE MODES OF ABRANGINO COLOURS IN SYSTEMS.

As we have seen in the previous chapters, the variety of
colours presented to us by nature and art is enormous, rang-
ing from the pure brilliant colours of the spectrum down to
the dull, pale tints of rocks and earth, and including whole
classes which seem at first glance to have but little affinity
with each other. It would be difficult, for example, to find
anything in the prismatic spectrum which reminds us in the
least of the colour brown ; the various pale tints which
wood assumes when worked seem quite unrelated to the
spectral hues ; and it is the same with the vast multitude
of greys with which we are acquainted, and which so large-
ly constitute the colours of natural scenery. If, instead of
comparing with the colours of the spectrum the strange,
wonderful, indescribable tints which nature so abundantly
displays, we descend a step and think of them in connection
with the hues furnished by our brightest pigments, such as
vermilion, red lead, chrome-yellow, emerald-green, or ultra-
marine-blue, we scarcely improve the matter : even such
colours as these seem to have no affinities with the modest
throng of greys and browns ; they appear to belong to a
more princely caste, and utterly refuse to mingle on equal
terms with the humbler multitude.

The colour of vermilion depends, as we learned some
time ago, on three things : first, on the quantity of coloured
light which it sends to the eye, or on the brightness of the

210 MODERN CHROMATICS.

coloured light that it reflects ; second, on the wave-length
of this red light ; and third, on the amount of white light
which is mingled with the red. Its colour depends, then,
on its luminosity, wave-length, and purity ; these quanti-
ties, as we have seen, are called the constants of colour.*
In certain cases we can easily alter these constants consider-
ably, and thus ascertain their significance with regard to
colour. Let us take a circular disk painted with pure ver-
milion, and undertake to reduce its luminosity. This could
be very efficiently accomplished by removing the disk to a
darkened room, when it would be found that the colour was
changed to dark red, or even to black. There is, however,
an objection to this mode of experimenting : we have al-
ways, under such circumstances, a strong tendency not sim-
ply to receive the tint that is actually presented to us, but
to make, unconsciously, an allowance for the degree of il-
lumination under which we view it. We know that red in
a darkened room presents a certain appearance ; when we
see this appearance in the darkened room, we say we see
dark red ; to the same appearance in a light room we would
give a different name. This objection applies to all experi-
ments of this character, whether the object be to expose a
surface to a very small or to a very great illumination : we
ourselves should, as it were, always be immersed in a medi-
um illumination, so as to retain an undisturbed judgment.
In the present case this difficulty is easUy avoided. We
take our vermilion disk to a room illuminated with ordinary
daylight, and reduce its luminosity by combining it with a
black disk, as indicated in Fig. 91. When the compound
black and red disk is set into rapid rotation, we in effect
spread out over the whole surface of the disk the small
amount of red light which is reflected from the exposed red
sector ; its luminosity can thus be reduced to any desii-able
extent. It will be found, when we combine 10 parts of

* See Chapter III.

MODES OF ARRANGING COLOtTRS IN SYSTEMS. 211

vermilion with 90 of black in this way, that the red colour
is converted into a chocolate-brown that bears no close re-
semblance to the original hue. It can be objected to this
mode of darkening colours that there may be in the black
disk something that exercises a peculiar effect on the result.
If, however, we analyze the faint light which comes from

Fra. 91. — Disk with 10 Parts Vermilion and 90 Parts Black : gives by rotation a dark-
reddish chocolate-brown.

the black disk with the aid of a prism, we shall find that it
is essentially white, all the prismatic colours being present.
Or we may expose our black disk to sunlight, analyze its
light with the prism, and compare this analysis with that
obtained from a piece of white paper not exposed to sun-
light and shaded from the diffuse daylight. When by this
proceeding the luminosities of the black and white papers
have been equalized, it will be found that the colours which
they furnish to the prism differ very little. The fact that
the black disk darkens the red one as it would be darkened
by a dark room can also be proved in another way : If we
first darken a room just as much as we please, and then
contrive an aperture opening into it a little larger than our
red disk, and arrange matters so that not much light enters
the room thi-ough this aperture, then we can virtually mix
the darkness of this room with the red light of our disk.
We simply place a red sector on the rotation machine ar-
ranged in front of the opening into the dark room, as indi-
cated in Fig. 92, and set the coloured sector into rapid

212

MODERN CHROMATICS.

rotation. We now have the red light of a small portion of
the red disk spread out over the darkness due to the dark-
ened room, and the result is the same : we have the same
chocolate-hrown (Fig. 93). The use of the black disk being
thus justified, we may employ it further in our investiga-
tion ; and we shall find that by simply reducing the red

light of our vermilion disk
with its aid, we produce not
only a series of dark, dull
reds, but a number of rich,
peculiar-looking browns.
When we make a corre-
sponding set of experiments

Tia. 92.— Eed Sector arranged with Dark
Koom as a Background.

Fig. 93. — Shows the appear-
ance presented -when the
sector of Fig. 92 is made to
rotate rapidly.

with our red-lead disk, we obtain a series of reddish, warm-
looking browns ; the chrome-yellow disk gives a set of
strange-looking, dull yellows and dark olive-greens ; the
green and blue disks furnish sets of dark green and blue
tones. Experiments like these show what changes are pro-
duced simply by reducing the intensity of the coloured
light, without in any other way tampering with it.

By a corresponding set of experiments we can study the
effects of mixing white light with our coloured light : we
need only combine the coloured disk with one that is white,
and rapid rotation gives us the desired result. In this way

MODES OF ARRANGING COLOtTRS IN SYSTEMS. 213

we can produce a series of pale tints that are reddish, green-
ish, or bluish, according to the disk employed.

Thus, either by reducing the luminosity of our'coloured
light or by mixing it with more or less white, a great num-
ber of tints can be produced ; but we soon find that to
match many natural colours it is necessary to employ both
these proceedings simultaneously. By combining with our
coloured disk a black and a white disk, we then become
able to imitate a far greater number of the pale, indescriba-
ble tints of natural objects. To make our power more
complete, we ought also to be able at will to alter the wave-
length of the light reflected from the coloured disk.* There
are, however, practical difSculties which prevent us from
making these changes in a definite and perfect manner, and
we find ourselves finally driven to abandon our very conve-
nient and instructive disks, and to turn for help to the col-
ours of the solar spectrum. These colours are pure in the
sense of being free from any admixture of white light, their
luminosity can be varied to any extent, and the lengths of
the various waves which generate them have been measured
with a high degree of accuracy ; we also can mix white
light with them at our pleasure. With the colours of the
spectrum, and a purple formed by mixing the red and violet
of the spectrum, we can match any colour whatsoever, pro-
vided we are allowed to increase or diminish the luminosity
of our spectral hues, and to add the necessary amount of
white light. This fact furnishes us with a clue toward a
classification of colours. The series red, orange, yellow,
green, blue, violet, and purple is one which returns on it-
self, and hence can be arranged in the form of a circle, as
was first done by Newton. In making a colour-chart we
can place the complementary colours opposite each other,
and white in the middle ; the pure colours of the specti-um
will be situated on the circumference of the circle, and the

* For an account of the effects produced by alteration of wave-length,
see Chapter III.

214 MODERN CHROMATICS.

mixtures of these with white will lie nearer the centre, as is
roughly indicated in Fig. 94. A chart of this kind will
contain all colours that are possible under a given degree of
illumination, arranged in an orderly manner. In the sector
devoted to the reds we shall find along the circumference
every kind of pure red, from purple-red to orange-red, and,
as we advance inward toward the centre of the circle, a
great number of tints produced by mixing these various
reds with increasing quantities of white. It is the same

Fig. 94.— The colours of the spectrum are supposed to be on the circumference of the
circle, and the mixtures of these with increasing qaantities of white are in the inte-
rior. White is at the centre. (The spaces for most of the pale or greyish tints are
left blank, for want of room.)

with all the other pure colours : every possible hue and
tint belonging to the adopted grade of illumination wiU be
found somewhere within the circle ; all the manifold greens
and blues, also the whole range of purples, from purplish-
red to purplish-violet— all will be represented. At the
start, one of our conditions was that complementary col-
ours should be opposite each other ; hence we must give to
our blue not only the right hue, but a luminosity such that
it is able exactly to neutralize the yellow which is opposite
to it. These two colours must also have a luminosity such

MODES OF ARRANGING COLOURS IN SYSTEMS. 215

that, when they are mixed, the mixture will furnish a white
which is twice as bright as that at the centre of the circle.
The same must be true of all the other pairs of complemen-
tary colours, whether situated on the circumference or in
the interior of the circle. This mode of operating gives us
a chart in which all the colours of corresponding intensity
are well arranged with regard to each other, and it enables
us at a glance to see some of the relations of the colours to
each other.*

In the construction of this colour-chart we imagined the
brilliant colours of the spectrum to be situated on the cir-
cumference of the circle, and as we advanced toward the
centre a larger and larger proportion of white light was to
be mixed with them. Suppose now we diminish somewhat
the luminosity of our spectral colours ; this will change
every tint in the chart correspondingly, and also the central
white ; all will be darkened proportionately ; we shall ob-
tain a new colour-chart, having less luminosity, but in other
respects closely resembling the first. With a further re-
duction of the light a corresponding result will again be
reached ; and, as we continue the process, we go on accumu-
lating new colour-charts, each being darker than the last.
Our two limits evidently are the brilliant colours of the
spectrum on the one hand and total blackness on the other ;
between these we shall be able to place a series of several
hundred colour-charts, differing perceptibly in luminosity,
but in other respects resembling the original type as nearly
as may be.

If we arrange this whole series of charts one above the
other in proper order, the most luminous one, or that with
which we started, being placed at the top, we shall obtain a
cylinder which will contain within itself an immense series
of colours. The axis of the cylinder at the top will be

* In this case we consider colours equally intense which, when mixed,
neutralize each other and produce the same white. The luminosity of such
colours, measured by the eye, will often be quite different.
10

216

MODERN CHROMATICS.

white, and as we descend it will pass through a gi-eat series
of darkening greys, finally to end in black. If we make a
vertical section of the cylinder, its appearance then will be
of the nature roughly indicated in Fig. 95, the axis darken-
ing as we descend, also the charts. Now, we know it to be
a fact that, as coloured surfaces are more and more feebly
illuminated, so does the number of tints which we can dis-

Fio. 95.— Section of a Pile of Colour-
Charts, the most luminous ones be-
ing at the top.

Fig. 96. — Section of a Colour-Cone.

tinguish on them constantly decrease. Hence, in the case
of our cylindrical pile of charts, smaller surfaces will an-
swer equally weU for the display of the darker tints, and
we may as well reduce our cylinder to a cone, as indicated
in Fig. 96. This colour-cone is analogous to the colour-
pyramid which was described by Lambert in- 1772.

It will be remembered that we began our colour-chart
with the colours of the spectrum, and these colours, along
with their mixtures with white, constituted the base of our
cone. The colours of the spectrum in this case were sup-
posed to have only a moderate degree of brightness, such
as would be suitable for prolonged observation. These are
the brightest colours thus far contained in our cone, but

MODES OF ARRANGING COLOURS IN SYSTEMS. 217

they are by no means the brightest colours that we are able
to see ; above them occur a great series of hues which we
can more or less perfectly distinguish. As our ability to
discriminate very luminous colours diminishes as their lumi-
nosity increases, the colour-charts devoted to this new set
may be treated like those used for the darker colours. In
this way we once more obtain a second cone, which will be
placed over the one before described. At the apex of this
last cone we have the brightest white which the eye is
capable of perceiving ; a little below and around it are sit-
uated a series of very luminous spectral colours and a pur-
ple, all being so bright as to differ, so far as the eye is
concerned, not much from a brilliant white ; farther down
on the sides of the cone are still quite bright colours, and
in its interior their mixtures with white. In this double
cone, then, we are at last able to include all the colours
which under any circumstances we are able to perceive ;
they are arranged in an orderly manner, which at once ex-
hibits their hues and luminosities, and the amount of white
light that has been mixed with them. They are arranged
throughout in complementary pairs, and some of their other
relations to each other are plainly exhibited.

Now, a word with regard to the possibility of executing
this colour-cylinder or the double cone. In the first place,
we have no pigments with which we can at all properly rep-
resent the colours of the spectrum even when their luminos-
ity is quite moderate ; our best pigments all reflect more or
less white light mixed with their coloured light. If with their
aid we undertook to construct a colour-chart, we should not
only be obliged to descend in the cone, Fig. 96, a good dis-
tance toward its black apex, but besides this our chart
would be smaller than the section of the cone at that point,
owing to the presence of the foreign white light reflected
by the pigments. It would be next to impossible to pre-
pare pigments of different colours suitable even for the pro-
duction of a single chart of the series ; for it would be ne-

218 MODERN CHROMATICS.

cessary that they should be right in the matter of hue,
luminosity, and greater or less freedom from white light.

There are still other objections to the system as just
proposed. It furnishes us with no means of giving the col-
ours a proper or rational distribution on the circumference
of the circle ; we do not know whether the yellow is to be
placed 90° from the red or at some other distance ; the
same is true with regard to the angular distribution of all
the other colours ; the system gives us no information on
this point. It also gives us no information about the effects
that are produced by mixing together colours that are not
complementary.

There is another mode of attacking this problem which
has been much used of late, and which offers certain advan-
tages to the student of colour. Let us suppose that we
place red of a certain luminosity at R, and green of the
same luminosity at G, Fig. 97 ; then along the line R G
we can arrange, or imagine to be arranged, all possible
mixtures of these two colours. To do this we imagine that
R and G have certain weights corresponding to their lumi-
nosities (or to the quantities of them which in a particular
case we employ), and then proceed as if we had before us

Ftg. 97.— Alonff the line E G we can arrange Fig. 98 —A mixture of equal parts

all the mixtures of red and green. The of red and green AimiBUeB a yel-

diagram represents the case where equal low; the position of this yellow on

parts of red and green are employed. the line of mixtures, li G-, is at Y.
Then the point of support (mixture-point)
must be in the centre of the line E (j.

a mechanical problem. An example will make this plain :
Let the luminosity of the red and green both be 10 ; we
now mix 5 parts of red with 5 of green, and obtain a yel-
low ; the position of this yellow will be at Y, Fig. 98, half

MODES OF ARRANGING COLOUES IN SYSTEMS. 219

way between R and G. We put the position of yellow at
Y because, in order that 5 parts of red may balance 5 parts
of green, the point of support must be half way between R
and G. We have now determined accurately on the line
R G the position of a yellow made by mixing equal parts
of our original red and green. This yellow, made by
mixing .5 parts of red and 5 of green, will also have the
same luminosity as the original red or green. If we mix 7
parts of red with 3 of green, the position of the mixture
will be at O, Fig. 99. If we divide up the line R G into

fl 0 G

rn — I — i — n i i i r i

Fta. 99.— Seven parts of red are mixed with three parts of green ; the mixture is orange
in colour, and situated on the line K G at O.

10 equal parts, then O will be distant from R by 3 parts,
but from G by 7 parts ; for in this way alone a balance can
be obtained. In general we shall always obtain a balance
or equilibrium when the weight of R multiplied by its dis-
tance from O is equal to the weight of G multiplied by the
distance of G from O. In this last case (Fig. 99) the mix-
ture at 0 will be orange in colour, and will have a lumi-
nosity identical with the original red and green. So we
can go on filling up the line R G with all possible mixtures
of the original red and green. Now, this is a process
which can actually be carried out in practice. "We can
take for our red a vermilion disk, and then select for our
green a disk having a colour as nearly as possible of the
same luminosity, and by the method of rotation produce all
the tints above indicated. We can copy these tints, and
arrange the copies along the line R G ; or, if we do not
wish to take this trouble, we can at least always reproduce
at will, with aid of the red and green disks, any of the
tints belonging on the line RG. This explanation will
serve to render clear the fundamental idea on which the
eolour-diagi'anj of Newton and Maxwell rests.

220 MODERN CHEOMATICS.

Thus far we have employed only two colours, red and
green, and have been able by mixing them in different pro-
portions to obtain various hues of orange, yellow, and
greenish-yellow. If we wish to include the other colours,
blue, violet, purple, and the mixtures of all the colours
with white, we must employ at the start three instead of
two colours. Maxwell selected vermilion, emerald-gi-een,
and ultramarine-blue, since according to his researches they
approximately represent the three fundamental colours.
These he placed at the three angles of an equilateral trian-
gle, and ascertained in a manner afterward to be explained
the position of white (or grey) in the interior of the trian-
gle. Every colour that can be obtained by mixing red
with green will lie on the line joining red and green ; it is
the same with green and blue, also with red and blue. Fig.
100 shows these colours disposed along the sides of the tri-
angle ; they are also so arranged that complementary col-
ours are opposite each other ; white is in the interior, and
along the lines joining the sides with the centre are placed
the various colours mixed with more and more white as
they are situated nearer to the centre. The colours made
by mixing red with green, or green with blue, being situ-
ated on the sides of the triangle, are consequently- as a- gen-
eral thing nearer to the position of white than the three
fundamental colours at the three angles of the triangle.
This indicates in a geometrical manner the fact that the
tints produced by the mixture of two fundamental colours
are paler, or mixed with more white, than the fundamentals
themselves. In general, in a chart of this kind, the farther
we go from the centre or from the position of white, the
more do we obtain colours which are free from admixture
with white. The angular position of the colours is to some
extent arbitrary, being determined partly by the process of
mixture, and partly by the assumption that particular hues
of red, green, and blue represent the fundamental colours.
If we should assume as our fundamental colours red lead,

MODES OF ARRANGING COLOURS IN SYSTEMS.

221

grass-green, and violet, the angular position of all the other
colours would be disturbed. Still, in spite of this and other
drawbacks, the colour-diagram is valuable for purposes of

U£

vwlet/

purp.violet/

,.-'''

\^CWN BLUE

/ RALE

purple/^

.efuE

\sff££W-SL(;E

carmine/ /'^^^ 'S)

^\\

RED

arELLOw

e/?££«

Fig. 100.— Shows tlio colours that are prodneed by mixing red with green, etc. The
pale colours, op those mixed with white, are situated in the interior of triangle, and
grow paler OS they are nearer W, or white.

study, and enables us to express our ideas about colour in a
geometrical form and with a certain degree of precision.
It is easy, for example, with its aid to ascertain the result
of mixing together any of the colours contained, or sup-
posed to l)e contained, in it ; if we mix equal parts of red
with cyan-blue, it is evident by inspection that the result
must be a whitish-purple ; so again equal parts of yellow
and cyan-blue will furnish a whitish-green. (See Fig. 100.)
"With the aid of this diagram we can accomplish even
we can mix together any number of colours, and

more ;

222 MODERN CHROMATICS.

ascertain the position and consequently the tint of the
resultant hue. We select any two, join them by a line,
and ascertain, as previously explained, the position of the
mixture tint ; we then join the third colour by a straight
line with the point just ascertained, and again construct
the position of the second mixture, and so on. An account
of the mode of making a colour-diagram of this kind wiU
be found in the appendix to this chapter, where the method
will also be explained which Maxwell employed for the
introduction of colours more or less luminous than the three
fundamental ones.

The colour-charts which thus far have actually been
published and laid before the world have been of a different
character from those above indicated, and are calculated to
display the effects, not of mixing coloured light, but col-
oured pigments. Among the older attempts we may men-
tion those of Le Blond in 1735, and of Du Fay in 1737. In
1758 T. Mayer published an account of his experiments.
As three fundamental colours he selected vermilion, a bright
yellow, and smalt-blue. Lambert in 1772 used for the con-
struction of his pyramid carmine, gamboge, and Prussian-
blue. These colour-charts were constructed by mingling
weighed portions of the fundamental pigments and of lamp-
black in such a manner as to obtain as great a variety of
tints as possible, which were then arranged in an orderly
series. The very beautiful colour-charts of Chevreul are
essentially of the same nature with those just mentioned.
Chevreul employed a circle with three radii which were
120° apart, and placed on these radii red, yellow, and blue ;
the hues of these colours were copied from certain portions
of the prismatic spectrum which were selected as standards.
Between red and yellow the various hues of orange and
orange-yellow were introduced ; between yellow and blue
the greens, the purples being situated between violet and
red. This constitutes the first chromatic circle, which con-
tains also the purest and most intense colours. In the sec-

MODES OF ARRANGING COLOURS IN SYSTEMS. 223

ond circle the same colours are shown mixed with a small,
definite amount of black ; the third circle is like the sec-
ond, only still more darkened, and so on. There are ten
of these circles, each containing seventy-two tints ; com-
plementary colours are situated opposite each other. Be-
sides the circles, there are charts containing colours arranged
in parallel bands ; these are intended to exhibit the effects
of mixing black and white with the colours contained in
the first circle. They consist of twenty-two bands, the 0
band being white, the 21st black, and the 10th band the
same as the corresponding colour in the first circle. Start-
ing, for example, from the 10th band, as we move to 0 the
colour grows continually paler, being mixed with more and
more white ; if we advance in the other direction, the col-
our becomes darker, and ends finally in black. There are
seventy-two sets of these bands, also one for black and
white.

The ideas upon which this ctart is based are not only
in the main arbitrary, but also vague, and the execution of
the sample examined by the author left much to be desired.
We can not regard this colour-chart as a true step toward
a philosophical classification of colours, but rather as a
more elaborate repetition of the work of Mayer, Lambert,
and Runge. In point of fact, our knowledge of colour
and our means of experimenting on it are not at present
sufSciently advanced to enable us even to propose a plan
for a truly philosophical classification, and between the pro-
posal and its execution there would be many weary steps.
Hence, the matter contained in this chapter and its appen-
dix is rather to be regarded as setting forth the problem
than as attempting its solution.

324

MODERN CHROMATICS.

APPENDIX TO CHAPTER XIV.

OOLOITE-DIAGEAMS.

FoLLOwma a suggestion of Newton's, Maxwell constructed a
diagram in which the colours of pigments and many natural objects
can be laid down in accordance with certain principles and assump-
tions presently to be explained. Now, although some of the as-
sumptions are arbitrary, yet if they are accepted a chart is obtained
which presents many valuable features for the student of colour.
The nature and scope of this colour-diagram will perhaps best be
made evident by tracing the actual construction of one as made by
the present writer.

Following Maxwell, vermilion, emerald-gi'een, and artificial
ultramarine-blue were assumed as the three fundamental colours,
and positions at the three angles of an equilateral triangle assigned
to them. The length of each side of the triangle was 200 divisions
of the scale employed. The first step was to determine the posi-
tion of white in the triangle. For this purpose disks of vermilion,
emerald-green, and ultramarine were combined as shown in Fig.
101, smaller central disks of black and white being placed on the

Fig. lOl.—Compoimd Disk of Vermilion, Emerald-green, Ultramarine, White and
Black, arranged for tile production of Grey.

same axis. It was found, when the colours were mixed by rapid
rotation, that 36'46 parts of vermilion, with 33-76 of emerald-green
and 29-76 of ultramarine, gave a grey similar to that obtained by
mixing 28-45 parts of white with 71-55 of black. In the experi-
ment 24-5 parts of white were actually obtained ; but this was cor-

APPENDIX TO CHAPTER XIV. 225

reoted by adding to it the white due to the black disk, it having
been previously ascertained that, if the luminosity of the white
paper composing the white disk was taken as 100, that of the black
disk was 5'24. The same correction was made in all the cases that
follow. The equation then reads :

36-46 R + 33-76 G + 29-76 B = 28-45 W (I).

The next step is to divide up the line K G, Fig. 103, in the ratio of
36-46 to 33-76 :

(36-46 + 83-76) : 36-46 :: 200 : 103-5.

That is, if we mix vermilion and emerald-green in the propor-
tion of 36-46 to 33-76, the mixture-point (a) lies on the line E G,
and is 103-5 divisions distant from G, Fig. 103. This point is the
position of the complement of the fundamental ultramarine, B.
We now connect this point (a) with B by a straight line, find the
length of the line to be 173-5 divisions, and seek the mixture-point
of 36-46 R + 33-76 G and 29-76 B, which is obtained by the follow-
ing proportion :

[(36-46 -I- 33-76) -|- 2976] : 29-76 :: 173-6 : 51-64.

Tills mixture-point is the position of white ; for vermilion, em-
erald-green, and ultramarine, when mixed in the above proportions,
produce white. Hence, white (W, Fig. 103) will be on the line
oB, 61-64 divisions from a. (It evidently must be somewhere on
this line, for at the two extremities of the line are colours which
are complementary, and there must be a mixture-point on the line
which is white.) It will be noticed that in this proceeding the
colours have been ideated, according to Newton's suggestion, as
though they were weights acting on the ends of lever-arms, and
these arms have been taken of such lengths as to bring the system
into equilibrium. It will also be observed that it has been assumed
that the pigments, vermilion, emerald-green, and ultramarine, have
the same intensity, or that equal areas of them have the same
weight. Thus, 36-46 i)arts of vermilion and 33-76 of emerald-green,
acting on a lever-arm 61-64 divisions in length, balance 29-76 parts
of ultramarine acting on an arm with a length of 121-86 divisions.
The lever-arms of the vermilion and emerald-green passing through
"W are also similarly balanced, and the whole system is in equilib-
rium.

The white or grey which was obtained in equation (1) was the

226 MODERN CHROMATICS.

equivalent of 100 parts of colour; by multiplying 28'45 by 3'51 we
obtain 100, and we set these 100 grey units in the place of 28-45 W
in equation (1), and obtain what Maxwell calls the corrected value
of the white. The factor 3'51 is called the coefficient of the white,
and is used to establish a relation between equation (1) and those
that follow. The coefficients of vermilion, emerald-green, and
ultramarine have at the outset been assumed as 1, and hence in its
corrected form equation (1) reads thus :

36-46 R + 33-76 G + 29-'76 B = 100 w (2).

We have now laid down upon our colour-diagram the position
of our three fundamental colours and that of white, and are pre-
pared to assign positions to all other pigments or mixtures of pig-
ments. For example, to determine the position of pale chrome-
yellow, a disk covered with this pigment is to be combined with
disks of emerald-green and ultramarine, and set in rotation (Fig.

riG. 102.— Oomponnd Disk of Chrome-yellow, Emerald-green, TTltramarbie, 'White, and
Black, arranged so as to give a pare Grey by rotation.

102). This experiment was made, and the following equation ob-
tained :

26-9 Y -I- 12-6 G + 60-6 B = 32-4 W -I- 67-6 b (3).

Before using equation (3), it is necessary to bring it into relation
with equation (2), and the first step is to express the value of the
white in the same manner as in equation (2), viz. . wo multiply it
by the coefficient 3-51, and obtain in this way the value of the cor-
rected white, or 113-87. This quantity we substitute in equation
(3), which then reads :

26-9 Y -t- 12-5 G -I- 60-6 B = 113-87 w (4).

APPENDIX TO CHAPTER XIV. 227

We must now introduce a corrected value for tlie chrome- yellow, so
arranged that we shall have the same number of units, grey or col-
oured, on both sides of the sign of equality :

IIS-ST — (60-6 + 12-5) = 40-77.

40-77 is then the corrected value of the chrome-yellow, and equa-
tion (3) In its corrected form finally reads :

40-77 Y + 12-5 G + 60-6 B = 113-87 w (6).

To obtain the coefficient of the chrome-yellow, we divide the cor-
rected value by the original value :

40-77

— — — = 1-51 = coef. of ch.-yel.

We are now prepared to determine the position of chrome-yel-
low in the diagram. We divide the line B G into two parts having
the ratio of 12-5 to 60-6 :

(12-6 -I- 60-6) : 12-5:: 200 : S4-2.

The position, then, of the complement of chrome-yellow is on the
line B G, Fig. 103, at the spot marked cobalt, and is distant from B
by 34-2 divisions ; the distance of this point from W is found by
measurement to be 94-1 divisions. We connect the point by a
straight line with W, and produce the line some distance beyond ;
the position of chrome-yellow will be on this line, and may be
found by the following proportion : Weight of chrome-yellow :
weight of emerald-green and ultramarine :: distance of emerald-
green and ultramarine : distance of chrome-yellow ; or,

40-77 : (60-6 + 12-5): -.94-1 : 168-7.

Clirome-yellow is consequently distant from the neutral point or
white 168-7 divisions; we insert it in the diagram along with its
coefficient 1-61. By a corresponding process the positions and coef-
ficients of a number of the more ordinary colours have been laid
down in the diagram. See Fig. 103. If the diagram is examined,
it will be found that along any single radius the pale colours, or
those mixed with much white, are located nearer W than those
that are more free from such admixture; it will also be noticed that
the more luminous colours have higher coefficients. By the aid of
this diagram we obtain relative measures of the luminosity and sat-
uration of colours on the same or on closely adjacent radii ; the

328

MODERN CHKOMATICS.

0.79

Fig. 103. — Maxwell's Colour- Diagram as constructed by 0. N. E.

colours also have angular positions assigned to them, so that they
are fairly defined as to angular position, intensity, and greater or
less freedom from white.

It is, however, to be remarked that the construction rests upon
several more or less arbitrary assumptions, as: 1. That vermilion,
emerald-green, and ultramarine-blue reaUy correspond to the three
fundamental colours. If we substitute in place of them other col-
ours, such as red lead, grass-green, and violet, we obtain different

APPENDIX TO CHAPTER XIT. 229

angular positions for all the colours afterward introduced, and also
different coefficients. 2. The assumption that vermilion, emerald-
green, and ultramarine have the same intensity or the same coeffi-
cient is quite unwarranted, the intensity of emerald-green heiag
evidently greater than than of ultramarine-blue. From this it fol-
lows that the coefficients and distances from the central white, W,
are not comparable along different radii.

The author has reconstructed the same colour-diagram, intro-
ducing coefficients which represent the actual luminosities of the
three fundamental colours. These coefficients were obtained by the
method described in Chapter III. Vermilion, emerald-green, and
artificial ultramarine-blue were, as before, assumed as the funda-
mental colours, and placed at the three angles of an equilateral tri-
angle. Taking the luminosity of white paper as 100, the luminosi-
ties of the three pigments were as follows : Vermilion, 26'85 ;
emerald-green, 48'68; ultramarine-blue, 7"57. Introducing these
coefficients into equation (1), it becomes :

Vermilion. Emerald-green. TJltramarine. White.

(26-85 X -3646) + (48-68 X -33Y6) -I- {1-51 X -2976) — 28-44.

That is :

9-8 verm. -1- 16-4 em.-green + 2-2 ult. = 28-44 white (6).

With the aid of equation (6), the position of white was determined
in the manner previously employed, and found to be that indicated
in Fig. 104, being only 13-55 divisions distant from the line V G.
Making use of equation (3), the coefficient of chrome-yellow was
determined as indicated below, X being this quantity :

(48-58 X -126) -1- (X X -269) + (7-57 x 0-606)= 32-4.
X = 80-82.

We then have —

6-072 em.-green + 21-74 eb.-yel. + 4-587 ult. = 32-4 (7).

With the aid of this last equation the position of chrome-yellow can
be laid down in the manner previously described. In Fig. 104 the
positions of the same pigments are shown which were employed in
the first colour- diagram, Fig. 103 ; and it will be noticed that white
has been moved toward the line R G, and that the angular positions
of the colours have been considerably altered. In this second col-
our-diagram the coefficients of pigments situated along different

230

MODERN CHROMATICS.
B„

S6. 1 REO LEAD

6REEN

s.S7.a

Fig. 104.— Maxwell's Diagram as reconstructed by the Author, correct Coefficients being

employed.

radii are comparable with each other, since they represent the lu-
minosities of the pigments compared with that of white paper taken
as 100.

The author has constructed a new kind of colour-diagram, in
which the colours are arranged in a different manner from those
just described.

Idea of the New Diagram,. — ^Let us suppose that we take a cer-
tain quantity of pure red light and locate it on the circumference of
a circle at E, Fig. 106, and draw the diameter K G- B, and at the
point G B locate a quantity of pure green-blue light, just sufficient
to neutralize the red light, or form with it a mixture which appears
to the eye white. The position of white will then be at the centre
of the circle, or at "W. The red and green-blue light employed will
be considered equal in intensity, though in actual luminosities they

APPENDIX TO CHAPTER XIV. 231

may differ considerably ; they will, in point of fact, relatively to
each other, have equal saturating powers. We next lay down on
the circumference at Y a certain amount of pure yellow light, draw
the diameter Y B, Fig. 105, and at B locate an amount of blue light

Fig. 105.

just sufficient to neutralize it, arranging matters so that the yellow
and blue light when mixed shall reproduce a white identical in
luminosity with that furnished by the mixture of the red and green-
blue. The yellow and blue will differ greatly in luminosity, but, as
they neutralize each other, will be considered to have equal inten-
sity. Each of the four colours will also be considered to have equal
intensity in the sense in which the word has just been employed ;
or, instead of using the term intensity, we may say that each of the
four coloui's will have corresponding powers of saturation. The
same will be true of any other colours belonging on the circumfer-
ence. In order to realize this idea, and to obtain means of assign-
ing to the colours proper angular positions, some other considera-
tions must be entertained. Suppose we mix the yellow located at

Y with the green-blue located at G B : we shall by varying the pro-
portions finally obtain a mixture which, although it is not white,
yet will be paler or more whitish than any other mixture ; this, of
course, is a well-known fact. In the practical construction of the
diagram, it is assumed that this most neutral mixture wiU be ob-
tained when the whole mass of the yellow at Y is mixed with the
whole mass of green-blue at G B; and it is evident that, even if
this assumption is not strictly true, it will approximate to the truth
just in proportion as the angular distance between R and Y hap-
pens to be a small quantity. If the angular distance between R and

Y is a large quantity, the assumption may or may not hold good ;

232 MODERN CHROMATICS.

at present we have no means of deciding this point. We will take
it for granted till the contrary is proved, and from n, the most neu-
tral point, we draw a perpendicular ; it will pass through the centre
of the circle, or through the position of white. The same wiU hold
good when any other point not far distant from R is connected by
a straight line with G B ; here also a perpendicular drawn from the
most neutral point will pass through white.

Bealization of the Diagram. — In order to construct this diagram
it is necessary to prepare three coloured disks having equal intensi-
ties, in the sense above employed, or equal saturating powers.
These disks must also have such colours that by optical mixture
they may be capable of furnishing white light. The colours select-
ed were red lead, a grass-green, and artificial ultramarine-blue.
The green disk was combined with the blue disk, and, by a rather
elaborate series of experiments, it was ascertained that the most
neutral mixture was obtained when equal areas were optically
mixed, from which it was concluded, according to the fundamental
assumption, that the saturating powers of the two disks were equal.
After several trials, a similar equality was established between the
green disk and one painted with shghtly darkened red lead. These
disks when combined gave the following equation :

23-06 red lead + 42-16 green -1- 34-'76 blue = 22-1 white.

The coefficients of the three colours were taken as unity, since the
colours had equal saturating powers. The relative areas of the col-
ours in the above equation were then used as weights, and farnished
the means of determining the positions of the three colours on the
circumference of a circle in which white was placed at the centre.
This was accomplished by placing the three colours at such angular
distances apart as brought the whole system into equilibrium ; for
example, if the weights had been equal, the angular distance of the
three points would have been 120°. The proper angular distances
being now laid down, the positions of darkened red lead, grass-
green, and ultramarine were determined; and with their aid the
positions of other pigments could be ascertained by the process of
mixture previously explained. (See Kg. 106.) The points laid down
in this diagram indicate colour or hue by angular position, and
saturation or intensity by greater or less distance from "W. The
relative amounts of white light reflected by the pigments situated
on any particular radius can easily be determined, since distance

APPENDIX TO CHAPTER XIV.

233

from the centre measures the amount of coloured light reflected,
and the total amount of coloured and white light reflected can be
measured by the process described in Chapter III. "We have, how-
ever, at present no means of generalizing this process and applying

Uir. BLUE
HAT.

CHROME YELLOW

Fia, 106.— SaturatlOD-Diagram according to Rood. The three colours Med in its con-
struction ore marked B.

it to colours situated on different radii, since we have not the power
of ascertaining, for example, whether our standard yellow disk at Y
reflects the same amount of white light with the standard red disk
at R, or more or less; we know that they reflect corresponding
quantities of coloured light, but nothing more. Before we can

234 MODERN CHROMATICS.

solve this problem it wiU be necessary for us to know the relative
luminosity of all the pure colours (free from white light) which, ac-
cording to the construction, fall on the circumference of the circle,
and this could only be ascertained by an especial study of the spec-
tral colours with reference to this point ; but no such study has yet
been made. We know that corresponding amounts of yellow and
greenish-yellow have not only higher degrees of luminosity than
their complements, blue and violet, but even higher than any of the
other colours; but thus far no quantitative determinations have
been made. Fig. 106 exbibits a diagram of the kind just described,
containing the same colours or pigments previously employed : it is
perhaps best called a saturation-diagram.

CHAPTER XV.

CONTRAST.

We have now studied with some care the changes which
coloured surfaces experience when viewed under various
kinds of illumination, or when modified in appearance by
the admixture of more or less white or coloured light. The
appearance which a coloured surface presents to us can,
however, be altered very materially hy a method which is
quite different from any of those that have thus far been
mentioned : we can actually change colour to a considera-
ble extent without at all meddling with it directly, it being
for this purpose only necessary to alter the colour which

BfD

GREEN

SfD

RCD

Fig. 107. — Sheets of Eed and Green Taper with Bed Squares.

lies adjacent to it. We can satisfy ourselves of this fact
by a very simple experiment. If we cut out of a sheet of
red paper two square pieces an inch or two in size, and then
place one of them on a sheet of red and the other on a sheet of
green paper, as indicated in Fig. 107, it will be found that
the red squai-e on the red paper will not appear nearly so

236

MODEKN CHROMATICS.

brilliant and saturated in colour as that placed on the green
ground, so that the observer will be disposed to doubt
whether the two red squares are really identical in hue.
By a somewhat analogous proceeding we can cause a sur-
face which properly has no colour of its own, which is real-
ly grey, to appear tinted red, blue, green, etc. These
changes and others of a like character are produced by
what is called contrast, and are partly due to actual effects
generated in the eye itself and partly to fluctuations in the
judgment of the observer. The subject of contrast is so
important that it will be worth while to make a somewhat
careful examination of the laws which govern it ; and for

Fig. 108.— Grey Paper with Green Slip.

Fig. 109.— Grey Paper with Eose-col-
oured Image.

this purpose it will be well for the reader to repeat some of
the simple experiments described below.

If we place a small piece of bright-green paper on a
sheet of grey drawing-paper, in the manner indicated in
Fig. 108, and then for several seconds attentively look at
the small cross in the centre of the green slip, we shall find,
on suddenly removing it, that in its place a faint image of
a rose-red colour makes its appearance, Fig. 109. This red
image presently vanishes, and the grey paper resumes its
natural appearance. The rose-red ghost which is thus de-
veloped has a colour which is complementary to that which
called it into existence, and this will also be the case if we

CONTRAST. 237

employ little squares of other colours : red will give rise to
a greenish-blue image, blue to a yellow, violet to a green-
ish-yellow, etc., the colour of the image being always com-
plementary to that which gave rise to it. On this account
these images are called negative, since, as far as colour
goes, they are just the reverse of the images which are first
presented to the eye of the observer. They are also often
spoken of in older treatises on optics as " the accidental col-
ours." It is quite easy to explain their production with the
aid of the theory of Young and Helmholt?. Let us take as
an example the experiment just described. Accordiag to
our theory, the green light from the little square of paper,
acting on the eye, fatigues to some extent the green nerves
of the retina, the red and violet nerves meanwhile not
being much affected. When the green paper is suddenly
jerked away by the string, grey light is presented to the
fatigued retina, and this grey light may be considered to
consist, as far as we are concerned, of red, green, and violet
light. The red and violet nerves, not being fatigued, re-
spond powerfully to this stimulus ; the green nerves, how-
ever, answer this new call on them more feebly, and in con-
sequence we have presented to us mainly a mixture of the
sensations red and violet, giving as a final result rose-red or
purplish-red. The green nerves, of course, are not so fa-
tigued that they do not act at all when the grey light is
presented to them, but the only effect that their partial
action has is to render the rose-coloured image somewhat
pale or whitish in appearance. The fatigue of the optic
nerve mentioned here does not differ essentially from that
which it undergoes constantly, even under the conditions of
ordinary use, where the waste is continually made good by
the blood circulating in the retina, and by the little inter-
vals of rest frequently occurring. In our experiment we
have merely confined the fatigue to one set of nerves, in-
stead of distributing it equally among the three sets.

The above experiments and explanation will enable us

238

MODERN CHROMATICS.

easily to comprehend the more complicated case, where, in-
stead of placing our little green square on grey, we lay, it
on a sheet of coloured paper. Instead, then, of grey, let us
take yellow paper, placing the green square on it as before,;
Fig. 110. On suddenly withdrawing the green square, we
find it replaced by an orange-coloured ghost, Fig. Ill,

YELLOW

aREEN

+ ♦

ORANaE

Fig. 110.-

-Yellow (

Jroraid -with

Green FiG. HI.— Yellow Ground with Or-
ange-coloured Image.

which we account for thus : As before, the green nerves
are fatigued, the red and violet nerves remaining fresh ;
when the square is removed, yellow light is presented to
the retina, and this yellow light, as explained in Chapter
IX., tends to act on the red and green nerves equally ; but
the green nerves in the present case do not respond with
full activity, hence the action is more confined to the red
nerves, and, as explained in Chapter X., the resultant tint
is necessarily orange — that is to say, we have a strong red
sensation mingled with a weak green sensation, and the re-
sult is the sensation called orange. In this experiment the
violet nerves do not come into play to any great extent. If
the green square is placed on a blue ground, the image be-
comes violet, for the reason that the blue light which is
presented to the fatigued retina acts, as explained in Chapter
IX., on the green and violet nerves ; but the green nerves
being fatigued, the action is mostly confined to the violet
nerves, and hence the corresponding sensation. In this
case the red nerves hardly come into play at all.

CONTRAST.

239

It follows from the above examples and reasoning that
the final effect is, that we obtain as an after-image what
amounts to a mixture of the complementary colour of the
-small square with the colour of the ground ; and, by recol-
lecting this, we can easily retain this class of facts in the
memory.

There is another similar experiment which is simpler
than those just described, but which is nevertheless instruc-
tive. A small square of black paper is to be placed on a
sheet of red paper, and the attention in this case is to be
directed to a mark on the edge of the former. (See Fig.
112.) When the black square is suddenly removed, the ob-

TiG. 112.— Eed Ground with Black Paper.

server sees in place of it a more luminous spot, which in
the case before us will of course be red ; but what is re-
markable is the circumstance that this red image will be
more intense or saturated in colour than the rest of the
ground. The rest of the sheet of red paper will look as
though grey had been mixed with its colour, Fig. 113.
This experiment will of course succeed with paper of any
bright colour, and Helmholtz has found that the same ef-
fects can be obtained with the pure colours of the prismatic
spectrum. The explanation, according to our theory, runs
about thus : While we are in the act of looking at the edge
of the black square, red light is passing into the eye, and is
11

240 MODERN CHBOMATICS.

fatiguing all those portions of the retina that are not pro-
tected by the presence of the black square ; it thus happens
that the ability of the larger portion of the retina to receive
the sensation of red is considerably diminished ; the ability
of the protected portion of course suffers no such change.

Fig. 118.— Eed Ground witU Intense Red Image

When the black square is suddenly removed, the unf atigued
portion of the retina receives a powerful impulse from the
red surface, but the effect produced on the rest of the retina
is inferior in degree. This accounts for the fact that the
image of the square is brighter or more luminous ; and we
can easily understand why it is at the same time more in-
tense, or saturated in colour, if we remember, as explained
in Chapter IX., that red light excites into action not only
the red nerves, but to a less extent the green and violet
nerves. Now, as the red nerves begin to be fatigued, the
action of the other two sets will be relatively more power-
ful than at first, so that gradually the sensations of green
and violet begin to add themselves to that of red (or, what
is the same thing, the sensation of white mingles itself with
that of red), and the red colour of the paper looks a little
greyish. The success «f the experiment with the pure col-
ours of the prismatic spectrum, which contain no white, is
easily accounted for by the explanation just given.

This matter can be pushed even further if, instead of

CONTRAST.

241

employing a black square, we take one which has a colour
complementary to that of the ground. We substitute then
for the black square one coloured with emerald-green, and
repeat the experiment. (See Fig. 114.) The result is much
the same as before, except that the red ghost is now still
more intense or saturated in colour, Fig. 115. When the

Fig. 114.— Blue-green Slip on Red Ground.

Fie. lis.— Intense Bed Image on Bed
Ground.

experiment is made in this way we accomplish two objects :
first, we protect a small portion of the retina from red light,
so that it may be very sensitive to this kind of light after-
ward ; second, we fatigue the green and violet nerves of
this portion by presenting to them bluish-green light, so
that afterward the red light from the red paper will be un-
able to stimulate them even in a small degree ; hence the
sensation that we receive is that of pure red, the action of
the green and violet nerves being excluded.

All these phenomena are cases of what is called succes-
sive contrast, because we look in succession from one sur-
face to another. When coloured surfaces are placed near
each other and compared in a natural manner, successive
contrast plays an important part, and the appearance of the
colours is more or less modified according to its laws. If
we attempt to confine our attention to only one of the col-
oured surfaces, this still holds good ; for the eye involun-
tarily wanders to the other, and to prevent this requires a

242

MODERN CHROMATICS.

good deal of careful practice, for fixed vision is quite op-
posed to our natural habit. It follows from this that, in
the natural use of the eye, the negative images, although
present to some extent, are not sharp and distinct, and
hence usually remain unobserved by persons not trained to
observations of this character. Nevertheless these images
modify to a considerable extent the appearances of coloured
surfaces placed near each other, and the changes of hue are
visible enough to the most uneducated eye.

One of the most common cases belonging here is repre-
sented in Fig. 116. We have a grey pattern traced on a

Tig. 116.— Grey Figure on a Green Gronnd.

green ground ; the tracery, however, will not ap1)ear pure
grey, but tinged with a colour complementary to that of
the, ground— that is, reddish. We can, of course, substi-
tute for the green any other bright colour, and it will al-

CONTRAST. 243

ways be found that the grey is more or less tinged with the
complementary hue. As black is really a dark grey, we
should expect to find it also assuming to some extent a col-
our complementary to that of the ground ; and this is in-
deed the case, though the effect is not quite so marked as
with a grey of medium depth. Chevreul, in his great work
on the simultaneous contrast of colours, relates an anecdote
which illustrates the matter now under consideration. Plain
red, violet-blue, and blue woven stuffs were given by cer-
taiii dealers to manufacturers, with the request that they
should ornament them with black patterns. When the
goods were returned, the dealers complained that the pat-
terns were not black, maintaining that those traced on the
red stuffs were green, on the violet dark-greenish yellow,
and on the blue copper-coloured. Chevreul covered the
ground with white paper in such a way as to expose only
the pattern, when it was found that its colour was truly
black, and the effects which had been observed were entire-
ly due to contrast. The remedy in such cases is not to em-
ploy pure black, but to give it a tint a little like that of the
coloured ground, taking care to make it just strong enough
to balance the hue generated by contrast. If we substitute
a white pattern for the black, something of this same effect
can often be observed, but it is less marked than with grey
or black. In cases like those now under consideration the
contrast is stronger when the coloured surface is bright and
intense or saturated in hue. The effect is also increased by
entirely surrounding the second colour with the first ; the
circumscribing colour ought also to be considerably larger
than its companion. When these conditions are observed,
the effect of contrast is generally noticeable only on the
smaller surface, the larger one being scarcely affected.

When, on the other hand, the two coloured surfaces are
about equal in extent, then both suffer change. If it is de-
sired to produce a strong effect of contrast, the coloured
surfaces must be placed as near each other as possible.

244

MODERN OHEOMATICS.

This is beautifully illustrated in one of the methods em-
ployed by Chevreul in studying the laws of contrast. Two
coloured strips were placed side by side in contact, as shown
in Fig. 117, duplicate strips being arranged in the field of

ULTe4M/IR;N£

CYAN BLUE

ULTRAMARmE:

CYAN BLUE.

Fig. 117.— Arrangement to show the Effects of Simultaneous Contrast, half size.

view at some distance from each other. The tints of the
two central strips were both altered ; those placed at a
greater distance apart suffered no change. In the experi-
ment represented in Fig. 117 the central ultramarine by
contrast is made to appear more violet in hue, the central
cyan-blue more greenish ; the colour of the outlying strips
is scarcely affected. In this experiment we have an appli-
cation of the rule above given for determining the changes
which colours experience under the influence of contrast.
The rule is quite simple ; its application, however, involves
a knowledge of the^ colours which are complementary to
each other, as well as of the effects produced by mixing to-
gether masses of coloured light. According to our rule,
when two coloured surfaces are placed in contiguity, each
is changed as though it had been mixed to some extent
with the complementary colour of the other. In the exam-
ple before us the ultramarine becomes more of a violet-blue,

CONTRAST.

245

because it is mixed, or seems to be mixed, with the comple-
mentary colour of cyan-blue — that is, ■with orange. The
cyan-blue appears more greenish, because it is virtually
mixed with greenish-yellow, which is the complementary
colour of ultramarine. As it requires a little consideration
to predict the changes which colours undergo through con-
trast, we give below a table containing the most important
cases :

Pairs of Colours. Change due to Contrast.

iEed Becomes more purplish.

Orange " " yellowish.

S Red " " purplish.

( Yellow " " greenish.

(Red " " brilliant.

( Blue-gi'CBU " " brilliant.

{Red " " orange-red.

Blue " " greenish.

j Red " " orange-red.

( Violet " " bluish.

3 Orange " " red-orange.

( Yellow " " greenish-yellow.

( Orange " " red-orange.

/Green :..■ " " bluish-green.

( Orange " " brilliant.

\ Cyan-blue " " brilliant.

( Orange " " yellowish.

\ Violet " " bluish.

( Yellow " " orange-yellow,

j Green " " bluish-green.

( YeUow " " orange-yellow.

] Cyan-blue " " blue.

( Yellow " " brilliant.

1 Ultramarine-blue " " brilliant.

1 Green " " yellowish-green.

/Blue " " purplish.

I Green " " yellowish-green.

/Violet " " purplish.

( Greenish-yeUow " " brilliant.

( Violet " " brilliant.

( Blue " " greenish.

I Violet " " purplish.

246

MODERN CHROMATICS.

It is easy and instructive to study the changes produced by
contrast with the aid of a chromatic circle, Pig. 118, and it

Fig. lis. — Chromatic Circle.

will be found that alterations in colour produced by con-
trast obey a very simple law : When any two colours of
the chromatic circle are brought into competition or con-
trasted, the effect produced is apparently to move them
both farther apart. In the case, for example, of orange
and yellow, the orange is moved toward the red, and as-
sumes the appearance of reddish-orange ; the yellow moves
toward the green, and appears for the time to be greenish-
yellow. Colours which are complementary are already as
far apart in the chromatic circle as possible ; hence they are
not changed in hue, but merely appear more brilliant and
saturated. This is indeed the effect which a strict applica-
tion of our rule leads to : the two colours are to be moved
farther apart ; they are already situated on the opposite
extremities of a diameter of the circle, and, if they are to
recede still farther from each other, they can accomplish
this in no other way than by moving outside of the circum-
ference of the circle ; but this corresponds, as explained in
the previous chapter, to an increase of saturation. If the

CONTEAST. 247

experiments indicated in tlie table are carefully repeated, it
will be found that all the pairs of colours there enumerated
are not equally affected by contrast. The changes of tint
are greatest with the colours which are situated nearest to
each other in the cbromatic circle, and much less with those
at a distance. Thus both red and yellow, are much changed
by contrast, the red becoming purplish, the yeUow greenish ;
while red with cyan-blue or blue is much less affected in
the matter of displacement or change of hue. On the other
hand, the colours which are distant from each other in the
chromatic circle, while suffering but slight changes in hue,
are made to appear more brilliant and saturated ; that is,
they are virtually moved somewhat outside of the circle,
the maximum effect taking place with colours which are
complementary.

Colours which are identical are affected by contrast in
exactly the opposite way from those which are complemen-
tary ; that is, they are made to appear duller and less satu-
rated. The author finds that these and other effects of
contrast can be studied with great advantage by the aid of
two identical chromatic circles laid down on paper. One
set of these lines should be traced on a sheet of transparent
paper, which is afterward to be placed over the companion
circle. The use of these circles will best be made evident
with the aid of an example. Let us suppose that we wish
to ascertain with their aid the effect produced by red, as
far as contrast goes, on all the other colours, and also on
red itself. We place the transparent circle on its compan-
ion, so that the two drawings may coincide in position,
and we then move the upper circle along the diameter join-
ing red and green-blue some little distance, so that the two
circles no longer have the same common centre. We then
transfer the points marked red, orange, yellow, etc., on the
upper circle, by pricking with a pin, to the lower circle, and
these pin-marks on the lower circle will indicate the changes
produced on all the colours by competition with red. Fig.

248

MODERN CHROMATICS.

119 gives the result. The stars on the dotted circle repre-
sent the new positions of the different colours when con-
trasted with red. If we examine them we find that red
when contrasted with greenish-blue causes this last colour
to move away from the centre of the circle in a straight
line ; hence, as the new point is on the same diameter, but
farther from the centre, we know that the greenish-blue is

NtVVWI

ORANGE

V GXmflNN

T^ GPEEXi

» GWESABUffi.

A- C!(M\HL\St

\l\3RMW«\Ut

Fig. 119.— Chromatic Circle displaced by Oontraet, showiDg the effects produced by the
red on th6 other colours.

not made more or less blue or green, but is simply caused
to appear more saturated or brilliant. The new point for
the red lies also on the same diameter, but is nearer to the
centre of the circle ; that is, the colour remains red, but
appears duller or less saturated. Experience confirms this.
If a considerable number of pieces of red cloth, for exam-
ple, are examined in succession, the last one will appear
duller and inferior in brilliancy to the others, but it will
still appear red. Proceeding with the examination of the
effects produced on the other colours, we find that orange
has been moved toward yellow, and also toward the centre
of the circle ; hence our diagram tells us that red, when put
into competition with orange, causes the latter to appear

CONTRAST. 249

more yellowish and at the same time less intense. Advanc-
ing along the circumference of the circle, our diagram in-
foims us that yellow is not much affected in the matter of
saturation or intensity, but is simply made to appear more
greenish. The two circles during superposition cut each
other near the position of yellow ; from this point onward
the effect changes as far as intensity or saturation is con-
cerned, the greenish-yellow being moved decidedly outside
of the original circle, as well as toward the green ; it is
made therefore, by contrast with red, to appear more bril-
liant as well as more greenish. Green is made to appear
somewhat bluish, and more brilliant. Greenish-blue has
been considered. Cyan-blue is made to appear slightly
more greenish as well as much more brilliant ; the same is
true of blue, though its increase in brilliancy by contrast
with red is rather less than in the case with cyan-blue.
Violet has its hue considerably altered toward blue ; its
saturation is diminished. Purple is made to look more violet,
and is much diminished in saturation. If we wish to study
the effects produced on the colours of the chromatic circle
by contrasting them with yellow, we have of course merely
to displace the .upper circle along the line joining yellow
and its complement ultramarine-blue, and then proceed as
before. The proper amount of displacement will of course
not be very large, and can be approximately determined by
experiment ; the upper circle, namely, is to be moved, so
that the colours situated on either side of the points where
the circles cut each other shall, in the diagram, Fig. 118, be
made to suffer changes of saturation corresponding to the
results of actual experiment.

It is quite evident that this contrast-diagram will fur-
nish correct results only on condition that the colours in it
are properly arranged ; if the angular positions of the col-
ours are laid down falsely, the results, in the matter of in-
crease or diminution of brilliancy or saturation, will also be
false. The author has made many experiments to settle

350

MODERN CHROMATICS.

Fig. 120. — Contrast-Diagram according to O. N. Rood.

this question, and in Fig. 120 gives tis result in the form
of a diagram ; the same result is given below in the form
of a table :

Table showing the Distances of the Colours from each other in the
Contrast-Circle, according to 0. N. R.

Angular Distances.

Pure red to vermilion , 6°

Vermilion to red lead 10'

Red lead to orange 9°

Orange to orange-yellow 36°

Orange-yellow to yellow 28°

Yellow to greenish-yellow 23°

Greenish-yellow to yellowish-green 13°

Yellowish-green to green 22°

CONTRAST.

251

Green to emerald-green 10°

Emerald-green to very greenish blue, or to complement of
carmine 18°

The hues of the papers employed in these experiments were
determined with some degree of accuracy by compaiing
them with a normal spectrum nearly six times as long as
that furnished by a single flint-glass prism, and at the same
time brilliant and pure. (See Chapter III.) The following
table gives the positions of these coloured papers in a nor-
mal spectrum, containing from A to H 1,000 equal parts ;
the corresponding wave-lengths are also given :

Coloured Papers.

Position in Nor-
mal Spectrum.

Spectral red (vermilion washed with carmine)

Vermilion (English)

Ked lead

Orange

Yellow (pale chrome)

Greenish-yellow

Yellow-green

Green

Emerald-green

Cyan-blue 2

Ultramarine, natural

Ultramarine, artificial

Violet ("Hoffmann's violet B.B.")

285
387
422
448
488
535
552
600
048
715
785
857

Wave-length in

6562
6290
6061
6000
6820
5649
5587
5411
5236
4991
4735
4472

Eather more reddish than
any violet in the spectrum.

From the foregoing, then, it is evident in general that
the effect of contrast maybe helpful or harmful to colours :
by it they may be made to look more beautiful and pre-
cious, or they may damage each other, and then appear
dull, pale, or even dirty. When the apparent saturation is
increased, we have the first effect ; the second, when it is
diminished. Our diagram. Fig. 119, shows that the satura-

252 MODEKN CRHOMATICS.

tion is diminislied when the contrasting cblours are situated
near each other in the chromatic circle, and increased when
the reverse is true. It might be supposed that we could
easily overcome the damaging effects of harmful contrast
by simply making the colours themselves from the start
somewhat more brilliant ; this, however, is far from being
true. The pleasure due to helpful contrast is not merely
owing to the fact that the colours appear brilliant or satu-
rated, but that they have been so disposed, and provided
with such companions, that they are made to glow with
more than their natural brilliancy. Then they strike us as
precious and delicious, and this is true even when the actual
tints are such as we would call poor or duU in isolation.
From this it follows that paintings, made up almost entirely
of tints that by themselves seem modest and far from bril-
liant, often strike us as being rich and gorgeous in colour ;
whUe, on the other hand, the most gaudy colours can easily
be arranged so as to produce a depressing effect on the be-
holder. We shall see hereafter that, in making chromatic
compositions for decorative purposes or for paintings, artists
of all times have necessarily been controlled to a consider-
able extent by the laws of contrast, which they have in-
stinctively obeyed, Just as children in walking and leaping
respect the law of gravitation, though unconscious of its
existence.

The phenomena of contrast, as exhibited by colours
which are intense, pure, end brilliant, are to be explained
to a considerable extent by the, fatigue of the nerves, as
set forth in the early part of the present chapter. The
changes in colour and saturation become particularly con-
spicuous after somewhat prolonged observation, and are
often attended with a peculiar soft glimmering, which
seems to float over the surfaces, and, in the case of colours
that are far af)art in the chromatic circle, to lend them a
lustrous appearance. Still, upon the whole, the effects of
contrast with brilliant colours are often not strongly marked

CONTRAST. 253

at first glance, from the circumstance that the colours, by-
virtue of their actual intensity and strength, are ahle to
resist these changes, and it often requires a practised eye
to detect them with certainty. The case is quite other-
wise with colours which are more or less pale or dark — that
is, which are deficient in saturation or luminosity, or both.
Here the original sensation produced on the eye is compara-
tively feeble, and it is hence more readily modified by con-
trast. In these cases the fatigue of the nerves of the retina
plays but a very subordinate part, as we recognize the ef-
fects of contrast at tlie first glance. We have to deal here
with what is known as simultaneous contrast, the effects
taking place when the two surfaces are as far as possible
regarded simultaneously. In the case of simultaneous con-
trast the changes are due mainly to fluctuations of the
judgment of the observer, but little to the fatigue of the
retiaal nerves.

We carry in ourselves no standard by which we can
measure the saturation of colour or its exact place in the
chromatic scale ; hence, if we have no undoubted external
standard at hand with which to compare our colours, we
are easily deceived. A slip of paper of a pale but very
decided blue-green hue was placed on a sheet of paper of
the same general tint, but somewhat darker and more in-
tense or saturated in hue. The small slip now appeared
pure grey, and by no effort of the reason or imagination
could it be made to look otherwise. In this experiment no
undoubted pure grey was present in the field of view for
comparison, and in point of fact the small slip did actually
approach a pure grey in hue more nearly than the large
sheet ; hence the eye iastantly accepted it for pure grey.
The matter did not, however, stop here. A slip of pure
grey paper was now brought into the same green field, but,
instead of serving as a standard to correct the illusion, it
assumed at once the appearance of a reddish-grey. The
pure grey really did approach reddish-grey more than the

254

MODERN CHROMATICS.

green field surrounding it, and hence was accepted for this
tint. The same pale blue-green slip, when placed on a pale-
reddish ground, assumed a stronger blue-green hue than
when on a white ground. In the first of these experiments!
we have an illustration of harmful and in the second ofl
helpful simultaneous contrast. The result in both cases co-
incided with that which successive contrast would have
produced under similar circumstances.

It has been stated above that the effects produced by
simultaneous contrast are dite not to retinal fatigue, but to
deception of the judgment ; now, as the effects of simulta-
neous contrast are identical in kind with those generated by

Fig. 121. — Shadow of Rod in Darkened Boom.

successive contrast, it is evident that the statement needs
some proof. This can be furnished with the aid of a beau-
tiful experiment with coloured shadows. In making this
experiment we allow white daylight to enter a darkened
room through an aperture. A, arranged in a window, as in-
dicated in Fig. 121. At R we set up a rod, and allow its
shadow to fall on a sheet of white cardboard, or on the

CONTRAST.

255

white wall of the room. It is evident now that the whole
of the cardboard will be illuminated with white light, ex-
cept those portions occupied by the shadow 1. We then
hght the candle at C, Fig. 122 ; its light will also fall on

Via. 122.— Shadows of Rod, using Daylight and Candle-light.

the cardboard screen, and will then cast the shadow 2 ; that
is, the candle-light will illuminate all parts of the screen
except those occupied by the shadow 2 ; this portion will
be illuminated with pure white light. Instead, however, of
appearing to the eye white, the shadow 2 will seem to be
coloured decidedly bliie. For the production of the most
powerful effect, it is desirable that the shadows should
have the same depth, which can be effected by regulating
the size of the aperture admitting daylight. Now, although
the shadow cast by the candle is actually pure white, yet,
by contrast with the surrounding orange-yellow ground, it
is made to appear decidedly blue. So strong is the illusion
that, even after the causes which gave rise to it have disap-
peared, it still persists, as can be shown by the following
experiment of Helmholtz -• While the coloured shadows are

256 MODERN CHKOMATIOS.

falling on the screen, they are to be viewed through a
blackened tube of cardboard, held in such a way that the
observer has both the shadows in his field of view ; the ap-
pearance then will be like- that, represented in Fig, 133.

Fig. 123.— Blue and Tollow Shadows viewed through a Tube.

After the blue shadow has developed itself in full intensity,
the tube is to be moved to the left, so that the blue shadow
may fill the whole field; The tube being held steadily in
the new position, the shadow will still continue to appear
blue instead of white, even although the exciting cause, viz.,
the orange-yellow candle-light, is no longer acting on the
eye. The candle may be blowil out, but the surface will
still appear blue as long as the eye is at the tube. On re-
moving the tube, the illusion instantly vanishes, and it is
perceived .that the colour of the surface is identical with
that of the rest of the screen, which is at once recognized
as white. In a case like this the fatigue of the retinal ele-
ments can play no part, as the illusion persists during a far
longer period of time than is necessary for their complete
rest ; we must hence attribute the result to a deception of
the judgment. Expressing this in the language of Young's
theory, we say that the sensation of white is produced
when the three sets of nerves, red, green, and violet, are

CONTRAST. 257

Stimulated to about the same extent ; but that nevertheless,
as we have in ourselves no means of judging with certainty
about this equality of stimulation, we may under certain
circumstances be induced to accept an unequal for an equal
stimulation, or the reverse. In the experiment with the
coloured shadows we had before us in the shadow due to
the candle-flame an equal stimulation, which by contrast
we were in the first instance induced to accept as unequal,
and the judgment afterward obstinately persisted in the
error till it was corrected and took a new departure.

This experiment may be modified and extended by the
use of coloured glasses instead of a candle-flame. The win-
dow is to be provided with two apertures, one of which is
to be covered with a piece of stained glass, through which
sunshine will be "admitted to the darkened room ; the other
aperture will admit white light, as before. If red glass be
employed, the colour of the shadows will appear red and
greenish-blue. In each case the shadows will assume com-
plementary colours.

The effects of simultaneous contrast can also be studied
with the aid of a contrivance of Ragona Scina. Two sheets
of white cardboard are attached to a couple of boards fast-
ened together at a right angle, as indicated in Fig. 124.
Between the boards a plate of rather deeply coloured glass,
G, is to be held in the manner shown in the figure, so that
it makes, with the vertical and horizontal cardboards, an
angle of about 45°. If the eye is placed at E, two masses
of light will be sent to it. From the vertical cardboard
white light will start, and, after being reflected on the glass
plate G, will reach the eye. This light will be white, or
almost entirely white, even after suffering reflection, owing
to the circumstance that, with a deeply coloured plate of
glass, the reflection takes place almost entirely from the
upper surface, or from that turned toward the light. The
second mass of light will proceed from the horizontal plate
H : originally of course it was white light, but on its way

358

MODERN CHROMATICS.

to the eye it traverses the glass plate, and becomes coloured
by absorption. If the glass plate is red, this light when it
reaches the eye will of coui-se have the same colour ; conse-
quently the first result is that we have presented to the eye

Fie. 124.— Apparatus of Bagona Scina for Contrast.

a mixture of red with white light, which will give the
observer the idea that he is looking at an horizontal, square
field of a somewhat pale reddish tint. If now a small
black square be attached to the vertical cardboard at B, of
course no white light can come to the eye from this portion
of the cardboard, and the image of this spot will seem to
the eye to be at b, on the -horizontal board under the eye.
Under ordinary circumstances this image would appear
black ; in point of fact, however, in this case it appears
deep red, owing to the red light transmitted by the plate
of glass. Thus far the arrangement amounts to a device
for presenting to the eye a mixture of red with white light,
the white light being absent at a certain spot, which conse-
quently appears of a deeper red. A similar black square
is now to be placed on the horizontal board at c ; it will of

CONTRAST.

259

course prevent the light from the place it covers from
reaching either the red glass or the eye, and under ordinary-
circumstances would be perceived simply as a square black
spot. Owing, however, to the fact that the upper surface
of the glass plate is reflecting white light to the eye, it
really appears as a grey spot. The final result is, that we
present to the eye at E a picture like that indicated by
Fig. 125 ; that is, on a pale-red ground we have a spot
which is pure grey, and near it one which is deep red.

PALE. RED

RED

GRIY

Fia. 125.— ColourB that are really presented to the eye in the eJiperiment of Eagona

Scina.

Owing to contrast, however, the appearance is different :
instead of a grey spot, we see one strongly coloured green-
blue (Fig. 126). This effect is partly due to contrast with

PALE RED

RED

SREEN
BLUE

Fio. 126.— Colours that are apparently presented to the eye,

the pale red of the ground, but still more to the presence
of the deep-red spot. This latter we can remove by taking
away the black square B, which diminishes the effect con-

260 MODERN CHROMATICS.

siderably. But now comes the most curious part of this
experiment : If we select a square of grey paper which has
the same color with the grey square seen in the apparatus
arranged as in Fig. 124 (apart from effects of contrast), and
place it over the glass plate and near the other two images,
it will not be afEected in colour, or only to a slight extent.
In point of fact, we now have, side by side, on the same
field, two grey squares quite identical in actual colour, but
one appears by contrast blue-green, while the other is not
afEected, but is perceived by the eye as being simply a
square of grey paper. As soon, however, as the observer
recognizes the fact that these two squares really have the
same grey colour, the illusion instantly vanishes, and both
of them remain persistently grey. It is evident that in
this case, as with the coloured shadows, the judgment is at
fault rather than the retLaal nerves ; for, as soon as an
opportunity offers, it corrects itseK and takes a new de-
parture. The illusion in this case, as well as with the col-
oured shadows, is produced quite independently of the
knowledge of the observer, who may indeed be a trained
physicist, minutely acquainted with the exact facts of the
case, and with all the details employed in producing the
deception, and still find himself quite unable to escape from
its enthrallment.

The simple experiments of H. Mayer are less trouble-
some than those just described, and at the same time highly
instructive. A small strip of grey paper is placed on a
sheet of green paper, as indicated in Fig. 127 ; it will be
found that the tint of the grey paper scarcely changes, tfti-
less the experimenter sits and stares at the combination
for some time. A sheet of thin semi-transparent white
paper is now to be placed over the whole, when it wUl in-
stantly be perceived that the colour of the small slip has
been converted by contrast into a pale red. Persons seeing
this illusion for the first time are always much astonished.
Here we have an experiment showing that the contrast

CONTRAST.

261

produced by strong, saturated tints is much feebler than
with tints which are pale or mixed with much white light ;
for, by placing tissue paper over the green sheet, the colour
of -the latter is extraordinarily weakened and mixed with a

Pre. 127.— Green and Grey Papers, for Experiment on Contrast ; one-fourth size.

large quantity of white light. In this experiment it often
happens that the red, which is due to contrast alone, seems
actually stronger than the green ground itself. If, instead
of using a slip of .grey paper, we employ one of black, the
contrast is less marked, and still less with one of white.
It is scarcely necessary to add that, if red paper is em-
ployed, the small grey slip becomes tinted by contrast
with the complementary colour, i. e., greenish-blue ; the
same is true with the other colours.

By preparing with Indian ink a series of slips of gi-ey
paper, ranging from pure white to black, an interesting
series of observations can be made on the conditions most
favourable for the production of strong contrast-colours.
The strongest contrast will be produced in the case of red,
orange, and yellow, when the grey slip is a little darker
than the colour on which it is placed, the reverse being
true of green, blue, viotet, and purple ; in every case, the
contrast is weaker if the grey slip is much lighter or much
darker than the ground. We must expect then, in paint-
ing, to find that neutral grey will be more altered by pale

262 MODERN CHROMATICS.

tints of red, orange, or yellow, which are slightly lighter
than itself, and that the grey will be less altered by these
colours when differing considerably from it in luminosity.
An analogous conclusion with regard to green, blue, violet,
and purple can also be drawn ; these colours should be
darker than the grey slip. Saturated or intense colours in
a painting have less effect on white or grey than colours that
are pale. This was shown in the preliminary experiment,
where grey was placed on a ground of strong colour. In
repeating these experiments, it will be noticed that the
effect of contrast is stronger with green, blue, and violet
than with red, orange, or yellow ; that is to say, it is
stronger with the cold than with the warm colours. If
now we reverse our mode of proceeding, and place a small
coloured slip on a grey ground, and cover the whole with
tissue paper, it will be difficult even with a green slip to
observe any effect of contrast. With a green sUp, one
sometimes imagines that the white sheet looks slightly
pinkish or purplish for an instant, but the effect is quite
uncertain. This is another illustration of the fact that, for
the production of strong effects of contrast, it is necessary
that the active colour should have a surface considerably
larger than the one to be acted on ; the former ought also,
if possible, to surround the latter. There is, however, a
limit beyond which this can not be carried. If the smaller
field is reduced too much in size, it is liable to melt to a
certain extent into the larger field of colour, in which case
we obtain, not the effect due to contrast, but that pro-
duced by a mixture in the eye of the two colours ; this is
indeed one method employed by artists for the mixing of
their colours.

Leaving now the contrast between pale colours and
pure grey, we pause to consider for a moment the contrast
of pale colours with each other. The laws governing this
species of contrast have already been explained in detail in
an earlier portion of the present chapter, and a construction

CONTRAST. 263

has been given by which the reader can study the differ-
ences produced in hue and saturation. To this it may now
be added that, where pale tints afe used in juxtaposition,
the phenomena are those of simultaneous contrast, the ret-
inal elements experiencing scarcely any fatigue ; hence the
effects are due to deceptions of the judgment, and occur
instantly. They are more marked than with intense or satu-
rated colours, and the effects produced even much more sur-
prising. These effects are heightened if the contrasted col-
ours have about the same degree of luminosity, or differ in
the same manner as in the spectrum ; that is, if the warm
colours are selected so as to be rather more luminous than
those that are cold. In Chapter III. the reader will find a
table showing the relative luminosity of the different col-
ours of the spectrum, and, what is still more to our purpose,
another giving the relative luminosity of the different com-
ponents of white light.

We must next examine the effects that are produced by
contrasting colours that differ in luminosity or in satura-
tion. If the two colours are identical except in the matter
of saturation, it will be found that the one which is more
saturated will gain in intensity, while its pale rival will
appear still paler. A slip of paper painted with a some-
what pale red, when placed on a vermilion ground, appears
still paler, and may actually be made to look white. If a
still paler slip be used, it may even become tinged greenish-
blue, its colour being in this case actually reversed by the
effect of contrast. With the aid of the two movable chro-
matic circles shown in Fig. 119, it is easy to trace these
changes theoretically. As a pale colour, or one mixed with
much white, already lies near the centre of the upper cir-
cle,* a small displacement carries it to the centre, that is,
makes it appear white ; or it may even transport it beyond,
and cause it to assume the complementary colour. When

* See Chapter XIV.

264 MODERN CHROMATICS.

the colours differ in luminosity, analogous effects are ob-
served. A dull-red slip was placed on a vermilion ground ;
the effect was as though' a quantity of grey had been added
to the slip ; it looked more dingy and somewhat blackish.
Another slip still darker and containing less red, when
placed on the same ground, looked as if it were tinged with
olive-green ; a still darker slip, with still less red colour,
treated in the same way, looked black with a tinge of blue ;
when, however, this last slip was placed on a white ground,
or compared with true black, it was seen that its colour
was far from black. The general result of contrasting col-
ours which differ much in strength, then, is that the feebler
one appears either more whitish or greyish, or assumes the
complementary tint ; the stronger one, on the other hand,
appears still more intense. If the strong and weak colours
are complementary to each other, then each of them gains
in intensity and appears purer, this gain seeming to be
greater in the case of the pale tint. From this it follows
that, while the juxtaposition of strong with feeble colours
usually injures or greatly alters the latter, colours that are
complementary furnish an exception, the reason of which is
evident at the first glance.

When the pale or dark colours are not complementary to
their more intense or brilliant rivals, they undergo the same
changes indicated in the table on page 245, the changes in the
case of the dull or pale colours being considerably greater.
In proportion as the colours are distant from each other in
the chromatic circle do they gain in saturation and beauty ;
while as they approach their character is altered, and they
are apt to look very pale, or, in the case of the dark col-
ours, blackish or dirty. This is particularly so when the
brilliant colour is large in surface and surrounds the darker
one ; with reversed conditions the effect is not so much
felt. Thus, a somewhat dull red near vermilion no longer
looks red, but brown ; a dull orange tint on the same
ground looks like a yellowish-brown.

CONTRAST. 265

It might be supposed from .what has preceded that col-
ours would enrich each other only when separated hy a
large interval in the chromatic circle ; and from a purely
physiological point of view this is indeed true. StUl, there
are other influences of a more spiritual character at work,
which modify and sometimes even reverse this lower law.
Thus, the presence of paler colour in a painting near that
which is richer often passes unperceived, simply making the
impression of a higher degree of illumination. We recog-
nize the representation of a flood of light, and delight in it,
without finding fault with the pale tints, if only they are
laid with decision and knowledge. Again, pale colour we
delight in as representing the distance of a landscape ; the
pale greenish-greys, bluish-greys, and faint tints of purple
which make it up, we never think of putting into envious com-
petition with the rich intense colours of the foreground, but
enjoy each separately, and rejoice in the effect of atmosphere
and distance, which neither kind of colour alone by itself
could adequately render. That is to say, for the sake of
light and atmosphere or distance, we gladly sacrifice a large
portion of the powerful tints at our disposal, and consider
ourselves gainers. The same is also true in another di-
rection ; we are ready to make the same sacrifice for the
sake of avoiding monotony and gaining variety, provided
only we can justify the act by a good reason. Cases of
this kind often occur in large masses of foliage, which, if
of the same general colour, are apt in a painting to look
monotonous and dull, unless great labour is bestowed in
rendering the light and shade and the small differences of
tint which actually exist in nature. Under such circum-
stances the observer feels a certain relief at the presence of
a few groups of foliage which are decidedly paler in colour
than the surrounding masses, provided only there is a good
excuse for their introduction. WiUow-trees agitated by
wind, and showing the under sides .of their leaves, which
are of a pale-greenish hue, offer a familiar example.

266 MODERN CHROMATICS.

Again, the mere contrast of dark or dull tints enhances the
colour and luminosity of those that are bright, and the ob-
server receives the impression that he is gazing at a mass
of gay and beautiful colouring, scarcely noticing the pres-
ence of the much larger quantity of tints that are darkened
by being in deep shade. These darkened shade-tints are
usually not variations of the same hue as the brighter tints,
but are more bluish ; so that, technically, the combinations
would often present instances of harmful contrast, were it
not for the fact that the bright and duU tints do not belong
even to the same chromatic circle, but to circles situated in
different planes, as explained in the previous chapter. Put-
ting this into more ordinary language, we should say sim-
ply that the strong contrast of light and shade masked such
effects of harmful colour-contrast as were present. There
is, however, another case where we are not so indifferent or
lenient. Where two objects are placed near each other in
a painting, and there is good reason why both should dis-
play the same colour with equal intensity, if one is painted
with rich colour, the other with a pale or dark shade of the
same colour, then the latter will look either washed out or
dirty, and a bad effect will be produced. As a familiar
illustration of this kind of effect, we may allude to the use
in dress of two widely differing shades of ribbon, which
have still the same general colour.

There is a still more general reason upon which the
pleasure that we experience from contrast depends. After
gazing at large surfaces fiUed with many varieties of warm
colour, skillfully blended, we feel a peculiar delight in
meeting a few mildly contrasting tints ; they prevent us
from being cloyed with all the wealth of rich colouring so
lavishly displayed, and their faint contradiction makes us
doubly enjoy the richer portions of the painting. So also
when the picture is mainly made up of cool, bluish tints :
it is then extraordinarily strengthened and brightened by a
few touches of warm colour. Precisely the same idea

CONTRAST.

267

holds good in drawings destitute of colour, and made up
merely of lines : if the drawing is composed mainly of soft
flowing curves, a few slanting straight lines across them
seem delicious ; or, to take a parallel case from another
department of art, in a discourse which is mainly grave in
tone the introduction of a few slight touches of humour
brightens and warms the audience most pleasantly. If
there is about an equal quantity of gay and sombre tints,
the effect is less good, becoming particularly bad when the
composition is divided up into many alternate groups of
gay and pale or sombre colours, impartially distributed.

Before concluding this chapter, it is necessary to add a
few words with regard "to the simple contrast of light and
shade, that is, where all the elements are comprised by
white, black, and intermediate shades of grey. As might
be expected from what has preceded, when a light gi'ey is

rio. 128.— Black and White Disk
for Exporlment on Contrast.

Fio. 129. — Showing the result when
disk, Fig. 12S, is set into rapid
rotation.

placed near a darker grey, the light shade appears still
lighter, the dark shade still darker. This can be beautifully
shown with the aid of our very convenient revolving disks.
"We take a black and white disk painted as represented in
Fig. 128. When this disk is set in rotation, it will, on ac-
count of the mixture of the black and white, produce a
series of grey rings, growing darker as we proceed from

268

MODERN CHEOMATICS.

the circumference toward the centre. Each ring, from the
circumstances of the case, will as a matter of fact haye an
absolutely uniform shade of grey, hut nevertheless it will
not appear so to the observer. The rings will appear to him
not uniform, but shaded, the lightest shade being always
turned inward toward the centre, as roughly represented in
Fig. 129. Where a ring comes in contact with one that is
lighter than itself, it is made to look darker ; with one
darker than itself, to look lighter. The same effect can be
observed by painting a series of pieces of paper with differ-
ent flat tints of grey, and then arranging them to corre-
spond with the disk experiment ; they will present an
appearance like that indicated by Fig. 130. It is hardly

Fia. ISO. — Slips of grey paper made to appear shaded by contrast.

necessary to add that in light-and-shade drawings, as well
as in nature, appearances of this kind, more or less modified,
are of constant occurrence. One of the most common cases
is where range after range of mountains rise behind one
another, the lower portions of the distant ranges appearing
lighter than the upper outlines. During rain, ranges of
hills often exhibit this phenomenon with astonishing dis-
tinctness. Even when the light and dark tints are at quite
a distance from each other, the phenomena of contrast pre-
sent themselves if there is much difference in the depth of

CONTRAST. 269

the two sets of shades. It is a very common experience
that the sky of a landscape in a drawing turns out too pale
after the rest of the drawing is completed. Contrasted
with the white paper of the unfinished sketch, it may look
quite right ; but, after the deeper tones of the distance and
foreground are added, may become quite insignificant.
Again, a few decidedly black touches in a drawing will
often by contrast lighten up portions that had previously
seemed considerably too dark ; or a few touches of pure
white will apparently darken spaces that had seemed pale
and weak. In each case the observer is furnished by exter-
nal means with a standard for measuring the depth of the
shade, and induced to use it rather than his memory. By
the skillful employment of contrast, drawings in light and
shade can be made to appear luminous and brilliant, or rich
and deep ; neglect of this element produces tameness and
feebleness. The contrast of light with dark shades is not
inferior in power to that of warm with cool tints ; and, in
point of fact, the contrast of white with black is the strong-
est case of contrast possible. We have on the one hand
the presence of all the colours, on the other their total ab-
sence. Hence, as has been noticed before, the contrast that
takes place between light and shade will sometimes mask
or even reverse that which occurs with different colours.
We can perhaps better tolerate a shortcoming in the matter
of colour-contrast than in that of light and shade ; if the
latter is right and powerful, we forgive a limited amount of
inferiority in the former, merely remarking that the work
is rather slight or pale in colour, but not on that account
pronouncing a verdict of total condemnation. On the other
hand, if the colour as such is right, but the depth of the
different tints mostly defective, then the whole is spoiled,
and we contemplate the tints, lovely enough in isolation,
with no satisfaction. We forgive, then, a partial denial of
the truths of colour more easily than those of light and
shade, which probably is a result of the nature of the opti-

270 MODERN CHROMATICS.

cal education of the race. For the human race, thus far,
light and shade has been the all-important element in the
recognition of external objects ; colour has played only a
subordinate part, and has been rather a source of pleasure
than of positive utility.

All that has been said with regard to the contrasts of
white, black, and grey, with slight modification, applies to
any single colour taken by itself ; for instance, to drawings
executed in one colour only, such as blue or brown. From
this it results that every colour is capable of exhibiting two
kinds of contrast, viz., that involved by competition with
other colours and that of mere light and shade.

The contrast of white, black, and grey with the series of
positive colours remains to be noticed. Taking up these in
order, we find that red when placed on a white ground ap-
pears darker and rather more intense in hue ; on a black
ground it becomes tinted somewhat orange-red, and looks
of course more luminous. Both these effects are probably
due ultimately to mere contrast of light and shade ; the
white ground makes the red by contrast look darker. But
we are accustomed to see red when it is darkened recede
from orange and approach pure red, or even perhaps to be-
come somewhat purplish ; hence it appears so in this case ;
it is an instance of expectant attention. When red is
placed on a black ground, it is made by contrast to look
more luminous ; but we are accustomed to see luminous red
become tinted with an orange hue ; hence the result. Red
on grey grounds of various depths undergoes modifications
corresponding to those just mentioned. Pale red, i. e., red
mixed with much white, on a white ground, gains in inten-
sity of colour ; on a black or dark-grey ground it loses
intensity, and approximates to pure white in appearance.
Here the contrast of light and shade is so strong as to cause
the colour to pass almost unperceived ; or we may say pale
red really does approach much nearer to pure white than
black, and hence is at last accepted for it. Dark, dull red

CONTKAST. 271

on a white ground may be mistaken for brown ; on a black
ground it appears more luminous and more red. Orange
on a wbite ground looks darker and more reddish, on a
black ground more luminous and yellow. The other effects
correspond with those described in the case of red. Tel-
low on a white ground appears darker and more greenish
than on a black ground ; in the latter case it is particu-
larly brilliant, and the black also looks well, taking on a
bluish tint. Dark yellow on a white ground looks brown
or greenish-brown ; on a black ground its colour is dis-
played to more advantage. Pale yellow on a white ground
is apt to look greenish, on a black ground to appear whitish.
Yellow and grey or black constitute a pleasant combina-
tion, of which extensive use has been made in nature and
art. Green on a white ground looks deeper and richer, on
a black ground somewhat paler ; by contrast the black is
made to look somewhat reddish or rusty. Green causes
grey to appear reddish ; the effect is particularly marked
when the grey has about the same luminosity with the
green, also when both are in the shade. Cyan-blue on
white appears darker and perhaps more greenish than on
black. Blue on white appears dark and rich, but shows no
tendency to green ; on black, by contrast, it becomes more
luminous. The same is true with blue on grey ; the latter
acquires a somewhat yellowish or rusty hue. The action
with violet is similar to that of bine.

From the foregoing it is evident that the contrast of
black, white, and grey with the colours depends mainly on
an apparent increase or diminution of their luminosity,
whereby in most cases their apparent hue is affected owing
to association. In the case of the colder colours, the tint
of the grey or black ground is affected, and shows a ten-
dency toward a hue complementary to the colour em-
ployed. We associate grey with blueness, and where the
effect is such as contradicts this habitual association it is
disagreeable ; on the other hand, grey with yellow forms

272 MODERN CHROMATICS.

an agreeable contrast, as tlie yellow tends to make the grey
look more bluish, and thus corrects any yellowish or rusty
appearance connected with it. It is claimed in some works
on colour that the complementary tints furnished by the
pure grey react on and strengthen the colours which call
them forth. An eye which is tired by gazing at green is
Ladeed rested by looking at its complement, i. e., at a mix-
ture of red and violet, and afterward will see the green with
more vividness ; but it is diflBcult to understand how the
presentation of red, violet, and green, or, what is the same
thing, grey light, can materially refresh the eye, or restore
its temporarily exhausted power. In the case of pale tints,
an effect of this character does indeed seem to take place,
but we must attribute it rather to an act of judgment than
to a physiological cause.

CHAPTER XVI.

ON THE SMALL INTERVAL AND ON GRADATION.

In the preceding chapter we have seen that, when two
colours which are nearly identical are contrasted, each is ■
made to appear less intense or saturated : red with orange-
red, yellow with orange-yellow, cyan-Wue with blue, are
examples of such combinations. From this it might be
supposed that, in chromatic compositions, it would not be
allowable to place colours thus nearly related in close juxta-
position. It is, however, found in practice that colours
which are distant from each other in the chromatic circle
by a small interval can be associated without detriment
under certain conditions. If the two colours express a varia-
tion in the luminosity of one and the same coloured surface,
they do not come into hurtful competition, and we receive
the impression of a single coloured surface, more highly
illuminated in certain portions. The scarlet coat of a sol-
dier when shaded appears red ; the sunlit portion is orange-
red. Grass in the sunshine acquires a yellowish-green hue ;
in the shade its colour is more bluish. But neither of
these cases produces on us a disagreeable effect, for we re-
gard them as the natural consequences of the kind of illu-
mination to which these objects are exposed. The effect is
not disagreeable even in mere ornamental painting, if it is
seen that the two tints are intended to express different
degrees of luminosity of the same constituent of the design,
even though this be only arabesque tracery. From this
explanation it follows that the two contiguous tints should

274 MODERN CHROMATICS.

have their luminosities arranged so as to correspond to
nature ; otherwise a contradictory effect would be produced.
The following table gives a series of small intervals, ar-
ranged properly as to luminosity; and it will be seen to cor-
respond to the relative luminosities of the colours of the
spectrum, or of the colours which taken together make up
white light (see Chapter III.) :

Table or Small Intervals.

Darker. Lighter.

Bed Orange-red.

Orange-red Orange.

Orange , Orange-yellow.

Orange-yellow Yellow.

Yellowish-green Greenish-yellow.

Green Yellowish-green.

Cyan-blue Green.

Blue Cyan-blue.

Ultramarlne-blue Blue.

Yiolet Purple.

Purple Red.

It will be noticed that the colours under the heading " Dark-
er " are really the shade-tints of the series opposite them,
and the difference may often be greater than that indicated
in the table. One of the commonest of these intervals is
that of yellow deepening into orange-yellow. In sunsets
yellow scarcely occurs without undergoing a change of this
kind ; it is almost the rule with yellow flowers ; and even
the pale, broken, subdued yellowish-browns of many natu-
ral objects manifest the same tendency. The relations of
greenish-yellow, etc., to green are shown beautifully by
foliage under sunlight, while the interval of cyan-blue to
blue or to ultramarine-blue is displayed on the grandest
scale by the sky. In brilliant sunsets the first and last pair
of intervals are of constant occurrence ; in fact, we can
scarcely think of a sunset without calling up in imagination

THE SMALL INTERVAL AND GKADATION. 275

red and purple. The interval greenisli-yeUow and yellow
is not included in the list ; it is perhaps less easy to tolerate
than any of the others ; we like to see yellow luminous or
rich, that is, passing into orange ; but, when it begins to
become decidedly greenish, we hesitate, unless there is
some good reason for accepting it. When the small inter-
val is used, and the two tints are put more or less into com-
petition by belonging to different surfaces, the effect is less
good, unless it is accounted for by the nature of the illumi-
nation, or in some other equally satisfactory way.

Much of the above applies to the case where the colours
pass into each other by gentle and insensible gradations, so
that the observer is quite at a loss to say where one ends
and the other begins. Here, as before, colours which are
nearly related, or separated only by a small interval, blend
harmoniously into each other and produce a good effect.
The reason of this is again found mainly in our precon-
ceived ideas of the changes which coloured surfaces under-
go when more or less strongly illuminated. If the colours
are quite distant from each other in the chromatic circle, a
rapid transition from one to the other, by blending, pro-
duces always a strange and often a disagreeable effect. A
yellow surface distinctly opposed or contrasted to a blue
surface often gives a good effect ; but, if it passes by a
series of quick gradations into blue, the effect is bad ; it is
as though it tried to assert at the same time that it was
warm and luminous as well as cold and dark. In the case
of the sky, it is true that we have, toward sunset, the yel-
low portions ^lending below into orange and red, and above,
by a long and slow series of gradations, into blue ; but the
distance between the blue and yellow is large, and they are
separated by a series of neutral tints, and we think of the
whole as an effect produced by apparent nearness or dis-
tance from the sun. Even in a case like this, many artists
prefer not to include in their paintings too much of the
upper blue, and thus are able to give more decided expres-

276 MODERN CHKOMATICS.

sion to the warmth and brightness of the sky. When we
see in nature a field of grass gradually growing decidedly
red, we think of clover as the excuse, without, nevertheless,
being particularly edified by its presence. In some moun-
tain lakes, such as the Konigssee, we find the blue-green
water actually passing in some places by rather quick gra-
dations into a purplish-red. The rapid transition into this
almost complementary hue produces an effect which seems
strange and almost incredible to those who for the first time
behold it. When the cause is recognized, we learn to look
upon the purple patches as marking the shallower water,
and, having accepted the effect as reasonable, we soon find
ourselves enchanted by it, and always remember it for its
strange beauty.

When two colours differing considerably, not only in
hue but in saturation, or simply in the latter respect, blend
rapidly into each other on the same surface, we always
require a reason for the change of tint ; and, if none is fur-
nished, the effect is apt to appear absurd, and resemble
somewhat the case of a man who at one moment is calm
and cool, and the next, without obvious reason, tender and
pathetic. When we find the cool grey or greyish-brown
tones on the surface of a cliff suddenly becoming rose-
tinted, we require an explanation of the change, and are
quite satisfied if told that the top of the cliff is still illumi-
nated by the sinking sun ; if, however, it is midday, we are
forced to think of red veins of some foreign substance dis-
seminated through the sober rock, and wonder what it can
possibly be, and wish it were away. All this forms one of
the minor reasons why painters like to keep their tints to-
gether in large masses, the bright warm colours in one
place, the cool pale tints in another.

One of the most important characteristics of colour in
nature is the endless, almost infinite gradations which al-
ways accompany it. It is impossible to escape from the
delicate changes which the colour of all natural objects un-

THE SMALL INTERVAL AND GRADATION. 277

dergoes, owing to the way the light strikes them, without
taking all the precautions necessary for an experiment in a
physical laboratory. Even if the surface employed be
white and flat, still some portions of it are sure to be more
highly illuminated than others, and hence to appear a little
more yellowish or less greyish ; and, besides this source of
change, it is receiving coloured light from all coloured ob-
jects near it, and reflecting it variously from its different
portions. If a painter represents a sheet of paper in a pic-
ture by a uniform white or grey patch, it will seem quite
wrong, and can not be made to look right till it is covered
by delicate gradations of light and shade and colour. We
are in the habit of thinking of a sheet of paper as being
quite uniform in tint, and yet instantly reject as insuflScient
such a representation of it. In this matter our unconscious
education is enormously in advance of our conscious ; our
memory of sensations is immense, our recollections of the
causes that produce them utterly insignificant ; and we do
not remember the causes mainly because we never knew
them. It is one of the tasks of the artist to ascertain the
causes that give rise to the highly complex sensations which
he experiences, even in so simple a case as that just consid-
ered. From this it follows that his knowledge of the ele-
ments that go to make up chromatic sensations is very vast
compared with that of ordinary persons ; on the other
hand, his recollection of mere chromatic sensations may or
may not be more extensive than theirs. Hence it follows
that it requires long training to acquire the power of con-
sciously tracing fainter gradations of colour, though much
of the pleasure experienced by their passive reception can
be enjoyed without previous labour.

These ever-present gentle changes of colour in all natu-
ral objects give to the mind a sense of the richness and
vastness of the resources of ITature ; there is always some-
thing more to see, some new evanescent series of delicate
tints to trace ; and, even where there is no conscious study

278 MODERN CHROMATICS.

of colour, it Still produces its effect on the mind of the be-
holder, giving him a sense of the fullness of Nature, and a
dim perception of the infinite series of gentle changes by
which she constantly varies the aspects of the commonest
objects. This orderly succession of tints, gently blending
into one another, is one of the greatest sources of beauty
that we are acquainted with, and the best artists constantly
strive to introduce more and more of this element into their
works, relying for their triumphs far more on gradation
than on contrast. The greatest effects in oratory are also
produced by corresponding means ; it is the modulation of
the tone and thought, far more than sharp contrasts, that is
effective in deeply moving audiences. We are very sensi-
tive to the matter of modulation even in ordinaiy speech,
and instantly form a general judgment with regard to the
degree of cultivation and refinement of a stranger from the
mode in which a few words are pronounced. All this has
its parallel in the use of colour, riot only in painting, but
also in decoration. Ruskiri, speaking of gradation of col-
our, says : " Tou will find in practice that brilliancy of hue
and vigor of light, and even the aspect of transparency in
shade, are essentially dependent on this character alone ;
hardness, coldness, and opacity resulting far more from
equality of colour than from nature of colour." In another
place the same author, in giving advice to a beginner, says :
" And it does not matter how small the touch of colour may
be, though not larger than the smallest pin's head, if one
part of it is not darker than the rest, it is a bad touch ; for
it is not merely because the natural fact is so that your col-
our should be gradated ; the preciousness and pleasantness
of colour depends more on this than on any other of its
qualities, for gradation is to colours just what cui-vature is
to lines, both being felt to be beautiful by the pure instinct
of every human mind, and both, considered as types, ex-
pressing the law of gradual change and progress in the
human soul itself. What the difference is in mere beauty

THE SMALL INTERVAL AND GRADATION. 279

between a gradated and ungradated colour may be seen
easily by laying an even tint of rose-colour on paper and
putting a rose-leaf beside it. The victorious beauty of the
rose as compared with other flowers depends wholly on the
delicacy and quantity of its colour-gradations, all other
flowers being either less rich in gradation, not having so
many folds of leaf, or less tender, being patched and veined
instead of flushed." *

All the great colourists have been deeply permeated by
a sentiment of this kind, and their works, when viewed
from the intended distance, are tremulous with changing
tints — with tints that literally seem to change under the
eye, so that it is often impossible for the copyist to say
exactly what they are, his mixtures never seeming to be
quite right, alter them as he will. Among modern land-
scape paintings, those of Turner are famous for their end-
less quantity of gradation, and the same is true even of his
water-colour drawings. The perfect blending of colours,
for example, in the sky, or in our best representations of it,
produces an effect of wonderful softness and beauty, the
tints melting into each other with a liquid smoothness for
which we can find no other parallel. The absolutely per-
fect gradation and softness of the sky well expresses its
qualities as a gas, impalpable, evanescent, boundless.

There is, however, another lower degree of gradation
which has a peculiar charm of its own, and is very precious
in art and nature. The effect referred to takes place when
different colours are placed side by side in lines or dots,
and then viewed at such a distance that the blending is
more or less accomplished by the eye of the beholder.
Under these circumstances the tints mix on the retina, and
produce new colours, which are identical with those that

* " Elements of Drawing," .by J. Ruskin. The distinguished artist
Samuel Colman once remarked to the writer, that this book not only con-
tained more that was useful to the student of art than any previous work,
but that it contained more than all of them put together.

280 MODERN CHROMATICS.

are obtained by the method of revolving disks. (See
Chapter X.) If the coloured lines or dots are quite distant
from the eye, the mixture is of course perfect, and presents
nothing remarkable in its appearance ; but before this dis-
tance is reached there is a stage in which the colours are
blended, though somewhat imperfectly, so that the surface
seems to flicker or glimmer — an effect that no doubt arises
from a faint perception from time to time of its constitu-
ents. This communicates a soft and peculiar brilliancy to
the surface, and gives it a certain appearance of transpar-
ency ; we seem to see into it and below it. Dove's theory
of lustre has perhaps some bearing on this well-known phe-
nomenon. According to Dove, when two masses of hght
simultaneously act on the eyes, lustre is perceived, provided
we are in any way made conscious that there are actually
two masses of light. On a polished varnished table we see
the surface by means of its imperfections, scratches, dust,
etc., and then besides have presented to us another mass of
light which is regularly reflected from the surface ; the
table looks to iis lustrous. The author, and afterward Dove
in a different way, succeeded in producing this lustrous ap-
pearance when only a single eye was employed, that is,
without the aid of binocular vision.* In the case before
us, the images of the colour-dots are more or less superim-
posed on the retina, and consequently seen one through the
other ; and at the right distance there is some perception
of a lack of uniformity, the degree of blending varying
from time to time. According to Dove's theory, we have
here the conditions necessary for the production of more or
less soft brilliancy. With bright complementary colours
the maximum degree of lustre is obtained ; when the col-
ours are near each other iq the chromatic circle, or dull or
pale, the effect is not marked, but exists to the extent of
making the surface appear somewhat transparent. When
the two colours are replaced simply by black and white,
* " American Journal of Science and Arts," May, 1861.

THE SMALL INTERVAL AND GRADATION. 281

the same lustrous appearance is still produced. Sir David
Brewster has described an experiment which has some hear-
ing on these matters. If a wall-paper is selected with a
pattern which repeats itself at intervals of a few inches, it
is possible, after some practice, so to arrange the eyes as to
cause the adjacent and corresponding portions to seem to
coalesce and form a new picture, which will in most respects
be identical with that obtained by ordinary vision. This
new picture will not seem to be at the same distance from
the eye as the real objects, and will move with each slight
motion of the head ; but what concerns us more is, that it has
a certain appearance of transparency and beauty not found
in the original. In this experiment two slightly dissimilar
masses of light are presented to the two eyes, and the result is
an appearance of transparency, using this word in its artis-
tic sense.

But to return : the result of this imperfect blending of
colours or of black and white by the eye is to communicate
to the surface an appearance of clearness, and to remove
any idea of hardness or chalkiness ; it is so familiar to us
that we accept it as quite natural, and only become con-
scious of its charm when it is withdrawn. As an example
in nature, we have the somewhat distant sea under a bright-
blue sky : the waves will be mainly green, the spaces be-
tween them blue ; these colours then blend into a sparkling
greenish-blue, which can not be imitated with a simple
mixed pigment. Also in grasses viewed at some distance,
the yellowish-green, bluish-green, reddish, purplish, and
brown tints, and the glancing lights, blend more or less to-
gether, and produce an effect which can not be reproduced
by a single sweep of the brush. The more distant foliage
of trees &n hillsides shows something of this kind ; and it
does not appear to be entirely absent even from the dust on
a traveled road, the minute sparkling grains of sand still
producing some action on the eye after they can no longer
be distinguished individually.

282 MODERN CHROMATICS.

In fresco-painting, and in scene-painting for the theatre,
most extensive use is made of this principle : at the right
distance adjacent tints hlend, and what near at hand seemed
a mass of purposeless daubs becomes an effective picture.
This same method of mixing colours on the retina of the
observer is also used more or less in oil painting with ex-
cellent effect ; it lends to them a magical charm, the tints
seeming purer and more varying ; the very fact that the
appearance of the painting changes somewhat according as
the observer advances or retires from it being an advantage,
communicating to it, as we might say, a certain kind of life.
Oil paintings in which this principle is not employed labour
under one quite demonstrable disadvantage : as the observer
retires adjacent tints blend, whether it was the intention of
the artist or not ; and if this has not been calculated for, a
new and inferior effect is pretty sure to be produced. In
water-colour drawings the same mode of working is con-
stantly employed under the form of stippling, more or less
formal ; and with its aid certain results of transparency
and richness can be attained, which otherwise would be out
of the reach of the artist. If the stippling is formal and
quite evident, it is apt to give a mechanical look to a draw-
ing, which is not particularly pleasant ; but properly used,
it has great value, and readily lends itself to the expression
of form. To descend several steps lower, we find the de-
signers of wall-papers and carpets employing this mode of
mixing colours and producing their gradations. In cash-
mere shawls the same principle is developed and pushed to
a great extent, and much of their beauty is dependent on it.
Finally, in etchings, engravings, and pen-and-ink drawings,
we have other examples of its application ; their clearness,
transparency, and sparkling effect being mainly due to the
somewhat imperfect blending of the black and white lines.
This effect can best be perceived by comparing them with
lithographs, or, better still, with Indian ink or sepia drawings.
The sky-like softness of the last two is very lovely, and, in

THE SMALL INTERVAL AND GRADATION. 283

some respects, very true to nature ; but if in them the line
manner is entirely avoided, they are a little apt to show a
lack of transparency in the deeper masses of shade, and to
look heavy or dull. This result is avoided by purposely
introducing a certain amount , of line-drawing, either with
the pen or a small brush. We have in natui-e a great
variety of appearances, and the various methods of art are
calculated to represent one or the other of them more or
less perfectly ; but there is no single kind of art manipula-
tion which will deal equally weU with all.

Finally, it is to be remarked that when colour is used
simply for ornamental purposes, blending or gradation
becomes of subordinate importance. This is the case, for
exmaple, where the design is worked out solely in flat tints.
Work of this kind, where the fancy is not allowed to inter-
fere much with the general correctness of the drawing or
colour, forms one of the first steps by which painting gradu-
ally passes over into pure ornamental design. We have
here colours arranged in harmonious masses, bounded by
sharp outlines, often definitely traced in black, and are
pleased with them, and with the beautiful, correct outlines.
All gradation and blending of colours is abolished, and this
fact alone announces to us, in an emphatic way, that the
design makes no pretension to realistic representation ; we
are pleased with the colours and outlines, and are rather
surprised to find how much can be accomplished by them ;
and if gold is introduced in the background or draperies,
its presence only adds to the general effect. By insensible
changes the figures of men and animals, etc., become more
conventional or grotesque, as in heraldry, until finally there
is no attempt made to portray any particular natural object.
Suggestions are taken from objects in nature, which are
used in much the same way fi-s by musical composers. The
intention, however, is the production of a beautiful design
which shall serve to ornament something else, as a woven
stuff, a vase, or the wall of a building. As gradation is

284 MODERN CHROMATICS.

one of .our most eificient modes of giving work in colour a
thorouglJy realistic appearance, it evidently can not be
much employed in ornament, where it is an object to avoid
any imputation of intentional realism.

In closing this chapter it may be well to allude to a
singular effect often produced by insensible gradation on
natural objects, or on their representations in paintings.
We have seen that a coloured surface having a well-defined
shape, when placed on a grey ground, is capable through
contrast of causing the ground to appear of the comple-
mentary colour. For example, a grey square or a green
ground will appear as though tinted with rose-colour. If,
however, the green passes into the grey by insensible gra-
dations, the matter may be so arranged that a small amount
of green causes the whole surface to appear green, when
most of it really is grey. This effect is often seen on rocks
partially covered with green moss : a few small patches on
the side exposed to the light will have a bright-green hue ;
some of the surface in the shade will be tinted dark green,
this colour passing gently into brown or grey, with here
and there a few quite small touches of olive-green. Three
fourths of the surface of the shaded side of the rock will
then be really grey or brown, but nevertheless the whole
wiU appear to be dark green. Another very common
example is furnished by the foliage of trees standing so
that the sun appears to be over them. Under these circum-
stances their tops and sides catch the sunbeams and appear
of a bright yellowish-green ; the rest of the tree is in the
shade, and appears at first sight of a darker green, and is
always so painted by beginners. If, however, the colour is
examined through an aperture about the size of a pea, cut
in a piece of white cardboard, it will be found that the real
colour is a somewhat greenish grey. On retiring farther
from the tree, this colour of its shady side will often change
to a pure grey, yet to a casual observer it will still appear
green. Quite wonderful effects have been obtained by

THE SMALL INTERVAL AND GRADATION. 285

some artists from the recognition of the principle here
involved ; their calm resignation of every trace of local
colouring, and acceptance in its place of some kind of grey,
imparting to their pictures a high degree of aerial perspec-
tive and of apparent luminosity.

CHAPTER XVII.

ON THE COMBINATION OF COLOURS IN PAIRS AND
TRIADS.

In the previous portions of this work we have dealt
with facts that are capable of more or less rigorous demon-
stration ; but we now encounter a great series of problems
that can not be solved by the methods of the laboratory, or
by the aid of a strictly logical process. Why a certain
combination of colours pleases us, or why we are left cold
or even somewhat shocked by another arrangement, are
questions for which we can not always frame answers that
are satisfactory even to ourselves. There is no doubt that
helpful and harmful contrast have a very great influence on
our decision, as will hereafter be pointed out ; but besides
this, we are sometimes influenced by obscure and even
unknown considerations. Among these may perhaps be
found inherited tendencies to like or dislike certain com-
binations or even colours ; influence of the general colour-
atmosphere by which we are surrounded ; training ; and
also a more or less delicate nervous susceptibility.

The author gives below, in the form of tables, some of
the results furnished by experience, and takes pleasure in
acknowledging his indebtedness to Brticke and to Chevreul
for much of the information contained in them.

Spectral i-ed * with blue gives its best combination.

Spectral red with green gives a strong but rather hard combination.

* A red between carmine and vermilion.

COMBINATION OF COLOURS IN PAIRS AND TRIADS. 287

Spectral red with yellow gires an inferior combination.
Spectral red with red lead gives a bad combination.
Spectral red with violet gives a bad combination.

If gold be substituted for the yellow pigment, the com-
bination becomes excellent. Red and yellow also make a
better combination when the red inclines to purple and the
yellow to greenish-yellow. The combination red and yellow
is also improved by darkening the yellow or both colours ;
this causes the yellow to appear like a soft olive-green (R.).
The combination red and green is also improved by darken-
ing both colours, or the green alone (R.).

Vermilion with blue gives an excellent combination.
Vermilion with cyan-blue gives an excellent combination.
Vermilion with green gives an inferior combination.
Vermilion with yellow gives an inferior combination.
Vermilion with violet gives a bad combination.

Vermilion and gold furnish an excellent combination.
The combination vermilion and yellow is improved some-
what by darkening the yellow ; if it is considerably dark-
ened, it tells as a soft olive-green (R.). Vermilion and
green are better when the green or both colours are much
darkened (R.).

Red lead with blue gives an excellent combination.

Red lead with cyan-blue gives an excellent combination.

Red lead with blue-green gives a strong but disagreeable combination.

Red lead with yellowish-green gives a tolerably good combination.

Red lead with yellow gives quite a good combination.

Red lead with orange gives quite a good combination.

Tlie combination red lead and bluish-green is improved
by darkening the green or both the colours (R.). Red lead
gives a better combination with a yellow having a corre-
sponding intensity or saturation ; if the yellow is too bright,
the effect is inferior (R.). The combination red lead and
yellow is much better than red and orange. The last two
13

288 MODERN CHROMATICS.

combinations given in tlie table are of course cases where
the small interval is employed. (See Chapter XVI.)

Orange with cyan-blue gives a good and strong combination.
Orange with ultramarine gives a good and strong combination.
Orange with green gives a good combination.
Orange with violet gives a moderately good combination.

Orange-yellow with ultramarine gives its best combination.
Orange-yellow with cyan-blue gives not quite so good a combination.
Orange-yellow with violet gives a good combination.
Orange-yellow with purple gives a good combination.
Orange-yellow with purple-red gives an inferior combination.
Orange-yellow with spectral red gives an inferior combination.
Orange-yellow with sea-green gives a bad combination.

Yellow with violet gives its best combinations.

Yellow with purple-red gives good combinations.

Yellow with purple gives good combinations.

Yellow with spectral red gives inferior combmations.

Yellow with blue, inferior to orange-yellow and blue.

Yellow with blue-green gives one of the worst possible combinations.

Yellow with green gives bad combinations.

The combination yellow and spectral red is improved by
darkening the yellow (K.). Blue-green and yellow, both
much darkeiied, give a better combination (R.). According
to Chevreul, yellow gives with green a good and lively
combination ; to this the author can not agree, although it
is true that the effect is improved by darkening the yellow
considerably. Chrome-yellow and emerald-green give com-
binations that are not bad when both the colours are very
much darkened (R.).

Greenish-yellow with violet gives its best combinations.
Greenish-yellow with purple gives good combinations.
Greenish-yellow with purplish-red gives good combinations.
Greenish-yellow with vermilion gives strong but hard combinations.
Greenish-yellow with spectral red gives strong but hard combinations
Greenish-yellow with red lead gives tolerably good combinations.

COMBINATION OF OOLOUKS IN PAIRS AND TRIADS. 289

Greenish-yellow with orange-yellow gives bad combinations.
Greenish-yellow with cyan-blue gives bad combinations.
Greenish-yellow with ultramarine gives a somewhat better combination.

The combination greenish-yellow and orange-yellow is
improved by darkening the latter colour, which then appears
brownish (R.). Greenish-yellow and cyan-blue make a bet-
ter combination when the blue is darkened (R.).

Grass-green with violet gives good but difficult combinations.
Grass-green with purple-violet gives good but difficult combinations.
Grass-green with rose gives combinations of doubtful value.
Grass-green with carmine gives combinations of doubtful value.
Grass-green with pink gives combinations of doubtful value.
Grass-green with blue gives combinations of doubtful value.

The value of the last four combinations is a disputed
matter. The combination green and carmine is improved
by darkening both colours considerably (R.). The combi-
nation green and blue becomes better as the green inclines
to yellow and the blue to violet (R.). The combination
green and violet, according to Chevreul, is better when the
paler hues of these colours are employed.

Emerald-green with violet gives strong but hard combinations.
Emerald-green with purple gives strong but hard combinations.
Emerald-green with red gives strong but hard combinations.
Emerald-green with orange gives strong but hard combinations.
Emerald-green with yellow gives bad combinations.

All these combinations are very difficult to handle.
Emerald-green and yellow, when both are much darkened,
furnish somewhat better combinations (R.).

Sea-green with vermilion gives good combinations.

Sea-green witli red lead gives good combinations.

Sea-green with violet gives good combinations.

Sea-green with purple-violet gives tolerably good combinations.

Sea-green with purple-red gives, simply as pairs, poor combinations.

290 MODERN CHROMATICS.

Sea-green with carmine gives, simply as pairs, poor combinations.
Sea-green with blue gives bad combinations.
Sea-green with yellow gives bad combinations.

The surface of the green should he much larger than
that of the vermilion or red lead.

Cyan-blue with chrome-yellow gives moderate combinations.

Cyan-blue with Naples-yellow gives good combinations.

Cyan-blue with straw-yellow gives good combinations.

Cyan-blue with carmine (light tones) gives good combinations.

Cyan-blue with violet gives poor combinations.

Cyan-blue with purple-violet gives poor combinations.

Cyan-blue with ultramarine gives good combinations (small interval).

The combinations of cyan-blue with violet and purple-
violet are not good, except in fine materials and light
tones.

Ultramarine with carmine gives poorer combinations than cyan-blue.
Ultramarine with purple-red gives poorer combinations than cyan-blue.
Ultramarine with violet gives, simply as pairs, poor combinations.

Violet with purple gives poor combinations if extended beyond the

small interval.
Violet with carmine gives poor combinations.

In studying the effects produced by colours in combina-
tion, it is of course important to exclude as far as possible
all extraneous causes that might influence or confuse the
judgment. Hence the colours under examination should be
disposed in very simple patterns, as the employment of
beautiful form or good composition might easily become a
means of leading the student to accept, as good, combina-
tions that owed their beauty to something besides mere
colour. For the same reason, gradation and good light-
and-shade effect should in such examinations be avoided ;
for these, as well as good composition, are means .of con-
cealing to some extent the poverty of a colour-combination.
For a similar reason the materials employed in such experi-

COMBINATION OF COLOURS IN PAIRS AND TRIADS. 291

ments should not be too fine. Almost any colour-combina-
tion worked out in stained glass appears pretty good, owing
to the brilliancy of the coloured light. This is one reason
why the patterns ia a kaleidoscope have been of so little
value in decorative art ; for, when the colours are most
carefully imitated in coarser materials, they are apt not
only to lose their brilliancy, but even sometimes to appear
dull or dirty from the effects of harmful contrast, which
did not make itself felt before. To a less degree this
applies also to silk ; many colour-combinations worked out
in this material are tolerable on account of its high reflect-
ing power, while the same colours, if transferred to wool or
cotton, appear poor enough.

In forming a judgment as to the value of combinations
of' colour, we should also be catitious in basing our conclu-
sions even on observations made directly from nature itself ;
for here our judgment is liable to be warped by the pres-
ence of beautiful form, good composition, exquisite grada-
tion, and high luminosity. Green and blue, for example,
make a poor combination, and yet it is one constantly
occurring in nature, as in the case where the blue sky is
seen through green foliage. This effect is often very good,
but a careful examination will show that, in most cases,
blue and green do not really come in contact ; for if the
sunlight penetrates the leaves in contact with the sky, they
no longer look green, but greenish-yellow, and this colour
makes a tolerable combination, particularly with ultramarine-
blue. Generally, however, the leaves actually in contact
with the sky are in the shade, or at least do not send bright
light to the eye, and we have really greenish-grey or
brownish-green combined with the blue of the sky. When
green actually does fairly touch the blue of the sky, as
with a forest of young trees growing thickly, the green is
usually far darker than the blue sky, as may be seen by
closing the eyes partially. Here the combination is helped
somewhat by light-and-shade contrast ; but when, owing to

292 MODERN CHROMATICS.

any cause, the blue of the sky is darkened till it approxi-
mates in luminosity to the green of the foliage, then the
colour-comhination is felt by an artist to be bad. The forms
of trees are so beautiful, the variety, the gradation they
exhibit so endless, the associations called up by them so
agreeable, that we are apt to deceive ourselves about this
colour-combination ; but when an attempt is made to trans-
fer it to canvas, we become painfully sensible of the fact
that Nature sometimes delights in working out beautiful
effects with colours that are of very doubtful value, cun-
ningly hiding their poverty with devices that often are not
easy to discover or to imitate.

There are several causes that may render a combination
of two colours bad. Prominent among them we find the
matter of contrast : the colours may look dull and poor on
account of harmful contrast, or may on the other hand
appear hard and harsh from an excess of helpful contrast.
The author has placed in the form of a diagram the results
of his observations on the effects of contrast in diminishing
or increasing the saturation or brilliancy of colours. This
diagram (Fig. 131) and its use are explained in Chapter
XV., and at present we merely remind the reader that
colours less than 80° or 90° apart suffer from harmful con-
trast, while those more distant help each other. In the
case of colours that are about 80° apart, the matter remains
a little doubtful ; the two colours may help each other
somewhat, or the reverse may be true. On comparing this
diagram with the results furnished by experience and given
in the preceding tables, it will be found that in good com-
binations the two colours are always more than 90° apart,
so that the effect of contrast is mutually helpful. Thus,
red furnishes good combinations with blue and cyan-blue,
which are considerably more than 90° distant from it ;
while the combination with artificial ultramarine, which is
nearer, is inferior, and that with violet bad. It does not
follow, however, that the colours in the diagram which are

COMBINATION OF COLOURS IN PAIRS AND TKUDS. 293

-cahmine"

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situated farthest apart always make the best combinations ;
for, if this were the case, the best combinations would be
simply the complementary pairs, which in the diagram are
placed at the greatest distance from each otber, viz., oppo-
site. But some of the complementary colours are quite
harsh from excessive contrast ; for example, red and its
complement green-blue, also purple and its complement
green. Of all the complementary pairs, according to
Briicke, these are least employed in art, as the harshness
with them is at a maximum. Now we can divide the
colour-diagram. Fig. 131, into two halves by a line drawn
from yellowish-green to violet, and the left-hand half will
contain the warm colours, the right-hand the cold. After
doing this, we find that red and green-blue, or purple and
green, are not only complementary, but also situated at or

294 MODERN CHROMATICS.

near the positions, if we may so express it, of the greatest
warmth and coldness ; hence, owing to a double reason, the
contrast becomes excessive, and the combination harsh.
According to the same authority, the complementary col-
ours most in use are ultramarine and yellow, blue and
orange-yellow, or cyan-blue and orange ; then follow violet
and greenish-yellow. In these cases, the complementary-
pairs are situated at some distance from the centres of
warmth and coldness, being in fact either on or not far
from the dividing line, which prevents excessive contrast
and the consequent hardness complained of in the examples
first cited. It may here be remarked that colours which
are truly complementary often appear better than those
which only approximate to this condition ; vermilion and
red lead, with their complements green-blue and greenish-
blue, do not furnish such offensive combinations as are
obtained when green is substituted for the true comple-
mentary hue.

The complementary colours are very valuable when the
artist is obliged to work with dark, dull, or pale colours,
and still is desirous of obtaining a strong or brilliant effect.
The fact that the colours are duU or pale or greyish pre-
vents much possibility of harshness ; and the use of com-
plementary hues excludes all risk of the brilliancy of the
tints being damaged by harmful contrast. In general, the
lower we go in the scale, and the more our colours approxi-
mate to black, brown, or grey, the more freely can we
employ complementary hues without producing harshness ;
and even those objectionable pairs, red and green-blue,
purple and green, if sufficiently darkened, become agree-
able.

It has been stated above that in good combinations the
colours are always a considerable distance apart in the
chromatic circle. This, however, does not exclude the class
of combinations mentioned in Chapter XVI.,- where it was
shown that any two colours differing but slightly produce

COMBINATION OF COLOUKS IN PAIRS AND TRIADS. 295

a more or less pleasant effect ; this is the case of the small
interval, which at present we are not considering.

Now, although in good combinations the colours are
rather far apart in the chromatic circle, it does not follow
that all colours that are far apart make good combinations.
When green, emerald-green, or bluish-green enters into a
combination, it is apt to produce a harsh effect if the green
is at all decided or covers much space. The enormous
difficulty of managing full greens or bluish-greens is per-
fectly well understood by artists, and many of them avoid
their use as far as possible. The presence in a picture of a
very moderate amount of a colour approaching bluish-green
or emerald-green excites in most persons a feeling of dis-
gust, and causes a work otherwise good to appear cold and
hard — very cold and hard. Corresponding to this, most
artists seem to be of the opinion that the pigment known
as emerald-green is more intense and saturated than any of
the other colours used by them. From a purely optical
point of view this would seem hardly to be the case :
emerald-green reflects more white light mixed with its
coloured rays than vermilion, and its luminosity is not out
of proportion to those of vermilion or ultramarine-blue, if
we adopt as our standard the luminosities of the corre-
sponding colours in the spectrum. Hence we must seek
elsewhere for the reason of its unusually intense action.
The author is disposed to attribute this well-known intol-
erance of all full greens to the fact that green light exhausts
the nervous power of the eye sooner than light of any other
colour. This exhaustion is proved by the observation that
the after-pictures, or accidental colours, are more vivid with
green than with the other colours. (See Chapter VEQ.)
Now, as a general thing, very strong sensations are offensive
when freely interspersed among those that are weaker ;
thus Hehnholtz has shown that discord in music is due to
the presence of " beats," which are merely rapid alterna-
tions of sound and silence following each other at such

290 MODEKN CHROMATICS.

intervals as to allow the sensitiveness of the ear to remain
at a maximum, and lience producing disagreeably intense
sensations which ofEend. Quite analogous to this is the
action of a flickering light, which is both disagreeable and
hurtful to the eye. This general principle, as it seems to
the author, applies also to the matter now in hand : a green
which optically may be the equivalent of a red, .yellow,
blue, or violet, nevertheless produces on the nerves of the
eye a more powerful and exhausting sensation than these
colours, and hence is out of harmony with them, or dis-
cordant. Besides this, and apart from these considerations,
green is not a colour suggestive of light or warmth, but is
what artists call cold ; the peculiar action above alluded to
renders it intense as well as cold, and consequently painters
are only able to employ it with a most cautious hand.
Yellow conveys thfe idea of light, red that of warmth : if
too much of either is present in a painting, the general
effect is of course impaired ; but by a small over-dose of
green, the picture is killed.

The colour which next to green acts most powerfully on
the nerves of the eye is violet ; after that follows blue-vio-
let (artificial ultramarine-blue). It so happens that, among
the pigments at the disposal of the painter or decorator,
violet has only a set of dull representatives ; hence it is not
quite so easy to transgress in this direction as with green,
for to obtain a violet which is at all the optical equivalent
of vermilion or emerald-green, it is necessary to use some
of the aniline colours. Blue-violet or artificial ultramarine-
blue easily gives rise to cold and hard combinations, and
large surfaces of it are apt to appear disagreeable if the hue
is at all intense. Skies painted with blues that are too in-
tense are easily ruined, and misjudgments in this direction
are not entirely confined to the work of beginners or ama-
teurs.

When the colours are arranged according to the order
in which they exhaust the nervous power of the eye, it is

COMBINATION OF COLOURS IN PAIRS AND TRIADS. 297

found that green heads the list ; violet, blue-violet, and
blue follow ; then come red and orange, and last of all yel-
low. This is also about the order in which we are able to
enjoy (or tolerate) positive colour in a painting ; large
masses of yellowish hues being often recognized only as
communicating luminosity, while hues of oi-ange or red-
orange, darkened, are called brown, and considered as
scarcely more positive than warm greys. From this it by
no means follows that the introduction of large masses of •
positive green into paintings is always to be avoided ; it is
not advisable, unless it- can be accomplished successfully,
and without injury to the work as a chromatic composition.
The ability to solve this problem in a brilliant manner is
one of the signs which indicate an accomplished colourist ;
and, when the green is combined with blue, the task be-
comes still more difficult and success more praiseworthy.
On the other hand, the handling of combinations of dull
yellow, brown, grey, or bluish-grey is much easier, and, in
fact, constitutes the first step by which beginners should
approach more positive colour.

As stated above, hurtful contrast is one of the common-
est reasons that render combinations of colour bad : as ex-
amples, we have orange and carmine, yellow and yellowish-
green, green and cyan-blue. All the colours in the con-
trast-diagram. Fig. 131, that are less than 80° or 90° apart
are more or less under the dominion of harmful contrast.
These effects become still more pronounced when the col-
ours have luminosities decidedly differing from those found
in the spectrum, where yellow is the brightest and violet
the darkest. (See Chapter III.) There are various modes
of mitigating to a considerable extent the effects of hurtful
contrast : a common one is to make one of the contending
colours darker than its rival, or to assign to it a much
smaller field ; a third colour situated at a considerable dis-
tance in the chromatic circle is also sometimes added.
Thus, for example, yellow and yellowish-green are im-

298 MODEEN CHROMATICS.

proved by adding to the conibination a small quantity of
violet or purplish-violet ; green and cyan-blue in the same
way are helped by the addition of purple or orange.
Harmful contrast in the matter of colour may also some-
times be concealed by strong light-and-shade effect, or by a
large amount of gradation, which tends to enable all col-
ours to maintain themselves against its influences ; beauty
and variety of form also to some extent mask its effects. It
may be added that apparent truth to nature sometimes
causes harmful contrast to be overlooked or pardoned ; on
the other hand, soiled or impure-looking tints, contradiction
of nature either in colour or form, and indecision'of hand-
ling, all are causes that intensify its action.

A combination may also be poor because the actual in-
tensities of the two colours differ too much, although then-
position in the chromatic circle is advantageous ; thus, for
example, the introduction of a quantity of chrome-yellow
into a design produces harsh effects, which would be avoid-
ed by the use of the more modest yellow ochre. When
this trouble exists in a high degree, the offending colour
usually catches the eye of the observer at first glance, and
before any of the other colours are fairly seen. A delicate
colour-emphasis is by no means easy of attainment, and its
lack produces on a chromatic composition effects quite analo-
gous to the want of the corresponding quality in speaking
or reading.

A combination may also be poor because it contains no
decided representative of the warm colours, including under
this term yellow and purple and the colours situated be-
tween them. There is reason to believe that the warm col-
ours actually preponderate in the most attractive and bril-
liant chromatic compositions ; however this may be, it is
certain that compositions founded almost exclusively on
the colder colours, such as yellowish-green, green, blue, and
violet, appear poor, and are apt to arouse in the mind of
the beholder a feeling of more or less dissatisfaction. The

COMBINATION OF COLOURS IN PAIES AND TKIADS. 299

general preference for warm colour is somewhat analogous
to that displayed for articles of food that have a tendency
rather to sweetness than the reverse ; but, however inter-
esting an inquiry as to the causes which during past ages
have brought about this result might be, it evidently would
not help us much in our present studies ; we are obliged to
accept the fact, and make as good use of it as our skill and
feeling for colour permit.

Thus far we have considered the effects that are pro-
duced when the colours are used in pairs ; they may, how-
ever, also be employed in triads. The studies that we have
made with the contrast-diagram. Fig. 130, render it easy
for us to select a series of triads that are free from the de-
fect of hurtful contrast ; for this will be the case with all
colours that are equally distant from each other in the dia-
gram, or are separated by an angle of 120° ; and, when we
examine the triads that have been most employed by artists
and decorators, we find that this principle has actually been
more or less closely observed. The triads that have been
most extensively used are :

Spectral red, yellow, blue ;
Purple-red, yellow, cyan-blue ;
Orange, green, violet ;
Orange, green, purple-violet.

In the second triad the colours are almost exactly 120°
apart ; in the first the yellow is a little less than 90° from
the red, and in fact forms with it a doubtful combination,
which is only rendered good by the presence of the blue.
In the third triad the orange and violet are about 90° apart,
but are nearly equally distant from the green, and form,
both of them, a good combination with it.

In the selection of colours for these triads a second prin-
ciple also seems to have guided the choice of artists : there
is an evident wish in each case that two out of the three
should be warm colours, and in two of the triads the matter

300 MODERN CHROMATICS.

of contrast has been somewhat sacrificed for the further-
ance of this end. The desire to satisfy both these condi-
tions of course greatly limits the number of triads, as an
examination of the contrast-diagram shows ; and, in point
of fact, in certain inferior triads which have been employed,
one or both of these principles have necessarily to a consid-
erable extent been neglected.

Carmine, yellow, and gi'een

was, according to BrUcke, a triad much used during the
middle ages, though to us the combination is apt to appear
somewhat hard and unrefined. Here we have two warm
colours, but the matter of contrast is also twice sacrificed ;
that is, slightly in the case of the carmine and yellow, and
more with the yellow and green.

Orange-yellow, violet, and bluisli-green

is an example of a combination which is poor not from defect
of contrast, but because it contains two cold colours, one of
them being the coldest in the chromatic circle.

Vermilion, green, and violet-blue

is a triad which has been extensively used in some of the
Italian schools. At first sight we have here apparently two
cold colours ; but, as the green was olive-green, the com-
bination really amounts to

Vermilion, dark greenish-yellow, and violet-blue,

and corresponds in principle with those above given.

In the employment of any of these triads in painting or
in ornament, the artist can, of course, vary the hue of the
three colours through the small interval without destroying
the definite character of the chromatic composition ; and
even small quantities of foreign colours can also be added.
When, however, they begin to assume importance in the
combination, they destroy its peculiar character. White or
grey can be introduced, and is often used with a happy
effect, particularly in the triads

COMBINATION OF COLOURS IN PAIRS AND TRIADS. 301

Orange, greea, violet ;
Purple-red, yellow, cyan-blue.

It is perhaps hardly necessary to d-well on the advan-
tages of studying the relations of colours to each other by
the use of pairs and triads, before more complicated arrange-
ments are attempted. Many of the pairs furnish 'opportuni-
ties for the construction of beautiful chromatic composi-
tions, and the practical study of colour in pairs and triads
can not be too strongly urged. In constructing a chromatic
composition, it is also of the first importance to determine
at the outset what the leading elements are to be ; after
this has been done, it will be comparatively easy to see
what variations are allowable, and what are excluded. The
most impressive and beautiful compositions are by no means
those that contain the most colours ; far more can be at-
tained by the use of a very few colours, properly selected,
varied, and repeated in different shades, from the most
luminous to the darkest.

We have now examined to some extent the good and the
poor combinations of colour, and it may be as well to add
a word with regard to the balance of colour ; for it is desir-
able that we should be able not only to select our colours
properly, but also to provide them in quantities suitable for
the production of the best effect. It has been a common
opinion among English writers on colour, that the best
result is attained by arranging the relative areas of the
colours in a chromatic composition in such a way that a
neutral grey would result if they all were mixed together.
It is quite true that, if the colours were portioned out in
this manner, there would be a balance of colour in an op-
tical sense, though how far balance in an aesthetic sense
would be attained is quite another question. Field in his
" Chromatics " has given certain rules for obtaining an op-
tical balance, and assumes that optical and aesthetic balance
are one and the same thing. For example, he states that if

302 MODERN CHKOMATICS.

we take red, yellow, and blue, of corresponding intensities,
then 5 parts of red, 3 parts of yellow, and 8 of blue will
neutralize eacb other in a mixture, and produce grey ; also,
8 parts of orange with 11 of green and 13 of purple will
produce the same result ; likewise 19 parts of citrine {" com-
pound of orange and green "), 21 parts of russet (" orange
and purple "), and 24 parts of a mixture of olive-green and
purple. These rules are based on the supposition that red,
yellow, and blue are fundamental colour-sensations, and
when mixed produce white, though, as we haye seen in
Chapter IX., this is quite the reverse of being true. In a
mixture of red, yellow, and blue, the yellow neutralizes the
blue, since these colours are complementary, and the super-
fluous red strongly tinges this grey or white light, which
then appears decidedly reddish. Field's actual experiments
on mixing colours were made by transmitting white light
through hollow glass wedges filled with coloured liquids ;
but it is, as we have seen in Chapter X., impossible in this
way to mix masses of coloured light. For example, the
light which passes through a yellow and a blue wedge placed
in contact is merely that which is not absorbed by either
wedge, or which both the wedges allow to pass. Both
wedges allow green light to pass, and stop almost all the
other rays ; but from this it is not allowable to draw, as
Fi^ld did, the inference that yellow light and blue light
make green light when mixed, since we know with the ut-
most certainty that these two kinds of coloured light make
grey or white light. Field's method gave entirely false re-
sults, and his conclusions based on them, including his so-
called " chromatic equivalents," have therefore for us neither
value nor meaning.

We return now to the proposition that the best effect is
produced when the colours in a design are present in such
proportions that a complete mixture of them would pro-
duce a neutral grey. It is very easy with our present
knowledge to ascertain what areas we must assign to two

COMBINATION OF COLOURS IN PAIRS AND TRIADS. 303

or more coloured surfaces in order to realize this effect. It
is only necessary to combine, according to Max-well's meth-
od, rotating disks which are painted with the pigments that
are to he used in the chromatic composition. Let us exam.-
ine this matter with the aid of a few actual examples.
Taking the first of the triads, spectral red, yellow, and blue,
we find that it is not possible to mix the colours in such
proportions as to obtain a neutral grey ; the yellow and
blue neutralize each other, and the red then colours the
mixture reddish. The same is true of the triad carmine,
green, yellow : the mixture will be orange, yellowish, or
greenish-yellow, according to the proportions. In the case
of the two triads, purple-red, yellow, cyan-blue ; and orange,
green, violet, neutralization can be produced by mixture ;
and, when the colours are thus arranged, the result is more
pleasing in the first than in the second case. If we take
triads not much used in art, we meet with similar results ;
for example, vermilion, green, ultramarine-blue, when com-
bined in such proportions as to furnish a grey, give a very un-
pleasing result, the cold colours being greatly in excess. But
it is needless to multiply examples, as the reader can easily
make these experiments for himself. If we examine the areas
and intensities of the colours in the works of good colourists,
we shall find that they are generally not such as to produce
neutrality when the colours are mixed ; that, on the contrary,
as in most of the above experiments, there is always an ex-
cess of some p(jsitive colour. The presence of this excess
gives a particular character to the composition, which will
vary with the hue which is thus emphasized. Hence we
see that this problem of the proper balance of colour is one
which can not be solved exactly by any set of rules, but
must be left to the feeling and judgment of the artist.

Attempts have been made from time to time to build
up theories of colour based on analogies drawn from sound.
The sensation of sound, however, is more particularly con-
nected with time, that of sight with space ; and these facts

304 MODERN CHROMATICS.

necessitate a fundamental difference in the organs devoted
to the reception of sound-waves and of light-waves ; and,
on account of this difference between the eye and the ear,
all such musical theories are quite worthless. Thus, our
perception for colour does not even extend over one octave,
while in music seven octaves are employed. When two
musical sounds are mingled, we have accord or discord, and
the ear of a practised musician can recognize the separate
notes that are struck ; but, when two masses of coloured
light are mingled, a new colour is produced, in which the
original constituents can not be recognized even by the eye
of a painter. Thus, red and green light when mixed fxir-
nish yellow light ; and this yellow is in no way to be dis-
tinguished from the yellow light of the spectrum, except
that it is somewhat paler and looks as though it had been
mixed with a certain amount of white light. Again, in
music the intervals are definite and easily recognized rela-
tions, as, for example, that of the fundamental with its fifth
or octave ; we can calculate the corresponding intervals for
coloured light, but they can not be accurately recognized even
by the most skillful painter. In painting we are constantly
obliged to advance from one colour to another by insen-
sible steps, but a proceeding like this in music gives rise to
sounds that are ludicrous. These facts, which are suscepti-
ble of the most rigid proof, may sufiice to show that a fun-
damental difference exists between the sensations of vision
and hearing, and that any theory of colour based on our
musical experience must rest on fancy rather than fact.

CHAPTER XVIII.

ON' THE USE OF COLOUR IN PAINTING AND
DECORATION.

The power to perceive colour is not one of the most
indispensable endowments of our race ; deprived of its pos-
session, we should be able not only to exist, but even to
attain a high state of intellectual and aesthetic cultivation.
Eyes gifted merely with a sense for light and shade would
answer quite well for most practical purposes, and they
would still reveal to us in the material universe an amount
of beauty far transcending our capacity for reception.
"But over and above this we have received yet one more
gift, something not quite necessary, a benediction as it were,
in our sense for and enjoyment of colour." * It is hardly
fair to say that without this gift nature would have ap-
peared to us cold and bare ; still, we should have lost the
enjoyment of the vast variety of pleasant and refined sensa-
tions produced by colour as such and by colour-combina-
tions ; the magical drapery which is thus cast over the vis-
ible world would have given place merely to the simpler
and more logical gi-adations of light and shade. The love
of colour is a part of our constitution as much as the love
of music ; it develops itself early in childhood, and we see
it exhibited by savage as well as cultivated races. We find
the love of colour manifesting and making itself felt in the
strangest places ; even the most profound mathematicians

* From an address by Professor Stephen Alexander,

306 MODERN CHROMATICS.

are never weary of studying the colours of polarized light,
and there can be no doubt that the attractive power of col-
our has contributed largely to swell the mathematical liter-
ature of this subject. The solar spectrum with its gorgeous
tints was for many years before the discoveries of Kirchhoff
and Bunsen a favourite, almost a beloved, subject of study
with physicists ; the great reward of this devotion was
withheld for nearly half a century ; divested of its colour-
charm, attracting less study, the spectrum might still have
remained an enigma for another hundred years.

Colour is less important than form, but casts over it a
peculiar charm. If form is wrongly seen or falsely repre-
sented, we feel as though " the foundations were shaken " ;
if the colour is bad, we are simply disgusted. Colour does
not assist in developing form ; it ornaments and at the
same time slightly disguises it : we are content to miss
some of the modeling of a beautiful face for the sake of
the colom'-gradations which adorn and enliven it.

The aims of painting and decorative art are quite diver-
gent, and as a logical consequence it results that the use
made by them of colour is essentially different. The object
of painting is the production, by the use of colour, of more
or less perfect representations of natural objects. These
attempts are always made in a serious spirit ; that is, they
are always accompanied by some earnest effort at realiza-
tion. If the work is done directly from nature, and is at
the same time elaborate, it will consist of an attempt to repre-
sent, not all the facts presented by the scene, but only certain
classes of facts, namely, such as are considered by the artist
most important or most pictorial, -,or to harmonize best with
each other. If it is a mere sketch, it will include not
nearly so many facts ; and finally, if it is merely a rough
colour-note, it will contain perhaps only a few suggestions
belonging to a single class. But in all this apparently care-
less and rough work the painter really deals with form,
light and shade, and colour, in a serious spu-it, the conven-

COLOtTE IN PAINTING AND DECORATION. SOT

tionalisms that are introduced being necessitated by lack
of time or by choice of certain classes of facts to the exclu-
sion of others. The same is true of imaginative painting : the
form, light and shade, and colour are such as might exist or
might be imagined to exist ; our fundamental notions about
these matters are not flatly contradicted. From this it fol-
lows that the painter is to a considerable extent restricted
in the choice of his tints ; he must mainly use the pale un-
saturated colours of nature, and must often employ colour-
combinations that would be rejected by the decorator.
Unlike the latter, he makes enormous use of gradation in
light and shade and in colour ; labours to express distance,
and strives to carry the eye beneath the surface of his pig-
ments ; is delighted to hide as it were his very colour, and
to leave the observer in doubt as to its nature.

In decorative art, on the other hand, the m.ain object is
to beautify a surface by the use of colour rather than to
give a representation of the facts of nature. Rich and in-
tense colours are often selected, and their effect is height-
ened by the free use of gold and silver or white and black ;
combinations are chosen for their beauty and effectiveness,
and no serious effort is made to lead the eye under the sur-
face. Accurate representations of natural objects are
avoided ; conventional substitutes are used ; they serve to
give variety and furnish an excuse for the introduction of
colour, which should be beautiful in itself apart from any
reference to the object represented. Accurate, realistic rep-
resentations of natural objects mark the decline and decay
of decorative art. A painting is a representation of some-
thing which is not present ; an ornamented surface is essen-
tially not a representation of a beautiful absent object, but
is the beautiful object itself ; and we dislike to see it for-
saking its childlike independence and attempting at the
same time both to be and to represent something beautiful.
Again, ornamental colour is used for the production of a
result which is delightful, while in painting the aim of the

308 MODERN CHROMATICS.

artist may be to represent sorrow, or even a tragic effect.
From all this it follows that the ornamenter enjoys an
amount of freedom in the original construction of his
chromatic composition which is denied to the painter, who
is compelled by profession to treat nature with at least a
fair degree of seeming respect. The general structure of
the colour-composition, however, being once determined, the
fancy and poetic feeling even of the decorator are com-
pelled to play within limits more narrow than would be
supposed by the casual observer. It is not artistic or sci-
entific rules that hedge up the path, but his own taste and
feeling for colour, and the desire to obtain the best result
possible under the given conditions. In point of fact, col-
our can only be used successfully by those who love it for
its own sake apart from form, and who have a distinctly
developed colour-talent or -faculty ; training or the obser-
vance of rules will not supply or conceal the absence of this
capacity in any individual case, however much they may do
for the gradual colour-education of the race.

From the foregoing it is evident that the positions oc-
cupied by colour in decoration and in painting are essen-
tially different, colour being used in the latter primarily as
the means of accomplishing an end, while in decoration it
constitutes to a much greater degree the end itself. The
links which connect decoration with painting are very
numerous, and the mode of employing colour varies con-
siderably according as we deal with pure decoration, or
with one of the stages where it begins to merge into
painting.

The simplest form of colour-decoration is found in those
cases where surfaces are enlivened with a uniform layer of
colour for the purpose of rendering their appearance more
attractive : thus woven stuffs are dyed with uniform hues,
more or less bright ; buildings are painted with various
sober tints ; articles of furniture and their coverings are
treated in a similar manner.

COLOUR IN PAINTING AND DECORATION. 309

The use of several colours upon the same surface gives
rise to a more complicated species of ornamentation. In
its very simplest form we have merely bands of colour, or
geometrical patterns made of squares, triangles, or hexagons.
Here the artist has the maximum amount of freedom in the
choice of colour, the surfaces over which it is spread being
of the same form and size, and hence of the same degree
of importance. In such cases the chromatic composition
depends entirely on the taste and fancy of the decorator,
who is much less restricted in his selection than with sur-
faces which from the start are unequal in size, and hence
vary in importance. After these simplest of all patterns
follow those that are more complicated, such as arabesques,
fanciful arrangements of straight and curved lines, or mere
suggestions taken from leaves, flowers, feathers, and other
objects. Even in these, the choice of the colours is not
necessarily influenced by the actual colours of the objects
represented, but is regulated by artistic motives, so that the
true colours of objects are often replaced even by silver
or gold. Advancing a step, we have natural objects, leaves,
flowers, figures of men or animals, used as ornaments,
but treated in a conventional manner, some attention, how-
ever, being paid to their natural or local colours, as weU as
to their actual forms. In such compositions the use of
gold or silver as background or as tracery, also the con-
stant employment of contours more or less decided, the
absence of shadows, and the frank disregard of local col-
our where it does not suit the artist, all emphasize the fact
that nothing beyond decoration is intended. Tip to this
point the artist is still guided in his choice of hues by the
wish of making a chromatic composition that shall be beau-
tiful in its soft subdued tints, or brilliant and gorgeous with
its rich display of colours ; hence intense and saturated
hues are often arranged in such a way as to appear by con-
trast still more brilliant ; gold and silver, black and white,
add to the effect ; but no attempt is made to imitate nature

310 MODERN CHROMATICS.

in a realistic sense. When, however, we go some steps
further, and undertake to reproduce natural objects in a
serious spirit, the whole matter is entirely changed ; when
we see groups of flowers accurately drawn in their natural
colours, correct representations of animals or of the human
form, complete landscapes or views of cities, we can be
certain that we have left the region of true ornamentation
and entered another which is quite diflferent. A great part
of our modem European decoration is really painting —
misapplied.

We return now to a brief consideration of monochromy,
or decoration in a single colour. In order to avoid the
monotony attendant on the use of a uniform surface of
colour, lighter and darker shades of the same hue are very
- often employed. These not only give more variety, but
serve also as a means of introducing various ornamental
forms, such as borders, centre-pieces, etc. Monochromy is
advantageously employed when it is desired, on the one
hand, to avoid the brilliancy attendant on the introduction
of several distinct colours, and on the other the dullness
consequent on the exclusive use of a single tone. It is
much used in wall-painting, also in woven stuffs intended
for articles of dress or for covering furniture, and for many
other purposes.

In monochromatic designs the small interval is very
frequently employed : for example, in using red, the artist
will employ for the lighter shades a red that is slightly
more orange than the general ground ; for the darker, one
that is rather more purplish. In this use of the small
interval, regard is to be had to the hues which colour
assumes under different degrees of illumination ; this mat-
ter is fully explained in Chapter XVII. Monochromatic
designs can furthermore be enlivened by ornamenting them
with gold, either alone or in connection with a small amount
of positive colour. The use of black and white is, however,
best avoided, as it furnishes occasion for the production of

COLOUR IN PAINTINU AND DECORATION. 311

contrast-colours which interfere with the general effect.
(See chapter on Contrast.)

In polychromy a number of distinct colours are em-
ployed simultaneously, with or without gold and silver,
white and black. The laws which guide the selection of
colours in this kind of ornamentation have already been
considered in Chapter XYII. Saturated and intense colours
are often used to cover only the smaller surfaces ; they are
then balanced or contrasted with colours of less intensity
spread over proportionately larger surfaces. In purely
decorative polychromy we deal mainly with rich and beau-
tiful arrangements of colour disposed in fanciful forms ;
natural objects, if introduced at all, being treated conven-
tionally. In the composition of such designs, however, the
artist is controlled to a considerable extent by the shape and
size of the spaces which the colours are destined to occupy;
the large masses of the composition in the best polychromy
being worked out in colours of proper intensity, which
make by themselves a broad design, over which again small-
er designs are wrought out in the same and in different
colours. As remarked by Owen Jones,* "The secret of
success is the production of a broad general effect by the
repetition of a few simple elements, variety being sought
rather in the arrangement of the several portions of the
design than in the multiplication of varied forms."

In the best polychromy great use is made of outlines or
contours ; they are employed to separate ornaments from
the ground on which they are placed, particularly when the
two do not differ greatly in colour. Colours that differ
considerably are prevented by contours, on the other hand,
from melting into each other and thus giving rise to mix-
ture tints ; in other words, each colour is made by the
separating outline to retain its proper position. Contours
when used for this purpose may be light or dark coloured.

* " Grammar of Ornament," London, 1856,
14

312 MODEKN CHROMATICS.

or even black. If the ornament is lighter than the ground,
the contour is made still lighter; if darker, the contour
mil he still darker. In the best decoration the figures of
men and animals, when introduced, are surrounded with
decided contours which emphasize the fact that realistic
representation is not intended. Contours are also made
white or golden ; they then become an independent part of
the ornamentation. Contours consisting of several lines of
gold and silver, white and black, are often used to separate
colours that do not harmonize particularly well together,
though, considered in a large way, they may still belong in
the compositions. These pronounced contours are never
intended to disappear when viewed at a distance, but form
a new ornamental element ; hence their shapes will often
vary more or less from the form of the spaces which they
enclose.

In the richest polychromy the designs are mainly worked
out in intense or saturated colours, along with gold and
silver, white and black. Dark and pa,le tints are not much
employed as such, but are produced by black or white
tracery on the coloured grounds. Corresponding to this,
variations of the dominant colours are effected, not by the
introduction of new tints, but by placing small quantities
of pure colour on a differently coloured ground ; the two
colours then blend on the retina of the observer and give
rise to the desired hue. For example, in a richly orna-
mented table-cover from Cairo, the writer noticed that the
use of a fine tracery of white on a blue ground gave rise to
the appearance of a lighter blue, which persisted at a dis-
tance ; in the border a pure red was made to appear orange-
red by a tracery of yellow ; in other portions, small red
and white ornaments on a blue ground produced at a dis-
tance the effect of a light violet tint.

In the superb decoration of the Alhambra, the colours
employed on the stucco work are red, blue, and gold ; pur-
ple, orange, and green are found only in the mosaic dados.

COLOUR IN PAINTING AND DECORATION. 313

The colours are either directly separated by narrow white
lines, or indirectly by the shadows due to the projecting
portions of the ornamentation. Masses of colour are never
allowed to come into contact. The blue and gold are often,
however, interwoven purposely, so as to produce at a dis-
tance a soft violet hue ; on this ground designs are traced
in gold and red, the gold figures being much larger than
the red ; or, on the same ground sometimes, wiU be found
figures in white with small touches of red. The principle
for the production of new colours above mentioned is con-
stantly employed : blue and white blend to a light blue ;
blue, white, and red furnish a light violet or purple hue ;
while red and gold mingle to a rich, subdued orange.
Sometimes in these designs the gold greatly predominates,
as in the " 5aU of the Ambassadors " or ia the " Court of
the Lions "; here we find a mass of wonderful gold tracery,
with only small portions of red and blue imbedded in it.
On the dados the mosaic designs are often worked out in
red-purple, green, orange-yellow, and a dark blue of but
slight intensity, the ground being grey. Narrow contours
of white separate the colours from the ground. To this
series light blue is sometimes added, or we find combina-
tions of orange-yellow, dark blue, and green or purple ;
dark blue and orange-yellow ; or simply orange-yellow and
small spaces of dark blue, the grounds in all these cases
being of a medium grey. The general effect of the colour
of the mosaics is jcool and somewhat thin ; it rests the eye
which has gazed on the magnificent displays placed above,
or prepares it by the contrast for new enjoyment.

True polychromy has not been very successfully culti-
vated in Europe since the time of the Renaissance, painting
having to a great extent usurped its place. Hence in mod-
ern times we find not only our porcelain, carpets, window-
shades, but the walls themselves and whatever else it may
be possible to decorate, covered with groups of flowers,
figures, or landscapes, architectural views, copies of cele-

314 MODERN CHEOMATICS.

brated paintings — all executed with as much pretended
truth to nature as the purchaser is able or willing to re-
ward. It is hardly necessary to add that the taste which
produces or demands such false decoration, while it may
have much to excuse, has but little to recommend it ; and
it is not to be expected that any general improvement can
be effected till the public at large learns better to distin-
guish between genuine decoration and genuine painting.

In decorative art the element of colour is more impor-
tant than that of form : it is essential that the lines should
be graceful and show fancy or even poetic feeling ; but we
do not demand, or even desire, that they should be expres-
sive of form in a realistic sense. Just the reverse is true in
painting : here, colour is subordinate to form. Neverthe-
less, its importance still remains very great, and it is trifling
to attempt to adorn with colour that which is really only a
light-and-shade drawing. The chromatic compositions of a
painting should from the start receive the most careful and
loving attention ; otherwise it is better to work in simple
black and white.

The links which connect designs in mere light and shade
with works in colour run about as follows : "We have, as the
fii'st step, pictures executed essentially in one tint, but with
endless small modifications. In this way a peculiar lumi-
nous glow is introduced which is never exhibited by designs
executed solely in black and white, or indeed in any one
tint. As examples of this kind of work we may mention
drawings in sepia or bistre, in which the tint is varied by
the introduction here and there of different quantities of
some other brown having a reddish, yellowish, or orange
hue. In the next stage the design is worked out essentially
in bluish and brownish tints. If a landscape, the distance
and much of the sky will be greyish-blue ; the foreground,
on the other hand, a rich warm brown, with here and there
a few touches of more positive color. The blue of the dis-

COLOUR IN PAINTING AND DECORATION. 315

tance will be variously modified, having often a greenish
hue, and being replaced in the more highly illujninated por-
tions by a yellowish tint. No real attempt will be made to
render correctly the natural colours of the objects depicted,
except as they happen to fall in with the system adopted.
By this mode of working, distance and luminosity can be
represented far more effectively than by the mere use of
black and white. Designs of this kind merge by insensible
degrees into others, where the strong browns of the fore-
ground vanish, and are replaced by a set of tints which,
though not . very positive, yet represent the actual colours
of the scene somewhat more truly. The rather uniform
bluish-grey of the distance, also, is exchanged for a greater
variety of cool bluish tints, and faint violet and purple hues
begin to mingle with the other colours. The yellows and
orange-yellows become more pronounced, but decided greens
are not admitted except in small touches, and as the local
colour requires it ; large masses also of any other strong
colours that happen to be present in the scene will be sug-
gested rather than represented. In designs of this kind
there is a good deal of room for the interchange and play of
different hues", and they make at first sight the impression
of being veritable works in colour. Many of Turner's earliffl-
drawings were executed in accotdance with these methods,
which allow the student gradually to encounter and over-
come the difficulties of colour. The substitution of paler
tints for the real colours of the scene, and particularly the
exclusion of green, a colour always difficult to manage, -
diminish the possibilities of entanglement in harsh or bad
combinations of colour, and render more easy the attainment
of harmony. This mode of using colour is of course con-
ventional, and pictures of this kind are not to be regarded
as executed in colour, in the full sense of the word. Among
genuine works in colour, the simplest are those painted es-
sentially with a single pair of colours, variously modified or
combined with gi"ey ; colours widely separated in the chro-

316 MODERN CHROMATICS.

matic circle from the selected pair being admitted only in
small masses or subdued tones. After these follow chro-
matic compositions in which three colours with their modi-
fications are systematically employed in the same manner,
to the exclusion as far as possible of all others. The char-
acter of these compositions will again vary according as the
light illuminating the scene in nature is supposed to be
white or coloured. If yellowish, the blue and violet hues will
be more or less suppressed, the greens more yellowish, while
the red, orange, and yellow tints will gain in intensity. Just
the reverse will occur under a bluish illumination. The
practice of employing an illumination of one dominant
colour, which spreads itself over the whole picture, modify-
ing all the tints, is very common among artists, and has
often been successfully used for the production of impres-
sive effects.

Good colour depends greatly on what may be called the
chromatic composition of the picture. The plan for this
should be most carefully considered and worked out before-
hand, even with reference to minor details ; the colours
should be selected and arranged so that they all help each
other either by sympathy or by contrast — so that no one
could be altered or spared without sensibly impairing the
general effect. No rules will enable a painter coldly to
construct chromatic compositions of this character ; the
constant study of colour in nature and in the works of great
colourists will do much, but even more important still is the
possession of a natural feeling for what may be called the
poetry of colour, which leads the artist almost instinctively
to seize on colour-melodies as they occur in nature, and af-
terward to reproduce them on canvas, with such additions
or modifications as his feeling for colour impels him to make.
Thus it is often advisable to deepen nature's colours some-
what, as in the case of the pale-tinted greys of a distance,
or in the mere suggestions of colour often presented by
flesh. In this process the proportions of the coloured and

COLOUK IN PAINTING AND DECORATION. 317

white light of nature are somewhat altered, and the col-
oured element made more prominent. On the other hand,
all the colours may be made paler and more greyish than
those of nature ; yet if they retain their proper relations,
if all are correspondingly affected, the harmony will not
be disturbed, and a design of this character will still be,
from a chromatic point of view, logical. If the cold hues, ,^-
the greens and blues, are allowed to stand in full strength,
while the warm colours, red, orange, and yellow, are weak-
ened, a particularly bad effect is produced.

Good colour, then, depends primarily on the chromatic
compositions ; next in importance on the drawing, includ-
ing under this term outline and light and shade. The want
of good, decided, and approximately accurate drawing is
one of .the most common causes that ruin the colour of
paintings. Powerful drawing adds enormously to the value
of the tints ia a coloured work when they are at all delicate,
or when the combination contains doubtful or poor colour-
contrasts, which in point of fact is a case common enough
in nature. Here the artist is obliged either to reject the
material furnished by nature, or to treat it in nature's own
way ; that is, the drawing must be excellent and the grada-
tion endless. Poor or bad combinations of colour are al-
most converted into good combinations by sufficient grada-
tion. When all the tints are pale, as in distances, it is
almost impossible to cause them to appear luminous or bril-
liant without the aid of delicate and accurate drawing.
There is still another way in which the drawing influences
the colour : perfectly clear, clean tints can be used, and will
look well, where the same colours in a slightly soiled or dirty
condition would be quite inadmissible. This results from
the circumstance that helpful contrast is favoured by clean,
even tints, while harmful contrast is strengthened by a dirty
or spotty condition of the pigments. This is peculiarly true
when the colours are not very positive, or are low in the
scale ; the tints, if not clear and decided, instantly lose all

318 MODERN CHROMATICS.

value and become a blemish. To insure this desirable ap-
pearance, called by artists purity, the colours must be laid on
rapidly and with decision, and not afterward gradually cor-
rected ; but to do this requires the hand of an accomplished
draughtsman.

The advance from drawing to painting should be grad-
ual, and no serious attempts in colour should be made till
the student has attained undoubted proficiency in outline
and in light and shade. Amateurs almost universally
abandon black and white; for colour at a very early stage,
and this circumstance alone precludes all chance of prog-
ress. The stage of advancement can, however, be very
easily ascertained. Thus, for example, if the student can
not execute a perfectly satisfactory study of any class of
subjects in outline with slight shade, then there is no use
in trying full light and shade ; if it is impossible for him
to draw the objects in full light and shade in a rather mas-
terly way, then there is no use in attempting colour. The
method employed by Turner of gradually effecting the
transition from black and white to colour has been just
described, and is worthy of the most careful study. In
making the first essays at colour, it is advantageous to exe-
cute careful studies of the scene in full light and shade,
but to note down the colours only in writing and in the
memory. Afterward, from these notes and the black and
white drawing, a colour-sketch may be attempted, away
from the scene. By this means fluctuations of judgment
about the colours and their relations are avoided, and,
though the painting may be all wrong, it has at least a
chance of being executed on one plan, and its frank errors
can afterward be ascertained. Beginners when working in
the presence of nature are apt to keep constantly altering
the plan of the chromatic composition, in the hope that it
will at last come right, and thus waste much time. Artists
under similar circumstances deliberately make up their
minds beforehand what colour-facts they will take, what

COLOUR IN PAINTING AND DECORATION. 319

view of the problem they will adopt, and adhere to this
decision unflinchingly.

After some progress has been made, the colour-sketches
that are attempted directly from nature should be simple
and executed with reference to colour, the element of form
being kept quite subordinate. The very natural desire to
make something that will afterward look like a picture is
to be suppressed, and the work performed rather with an
eye to the remote future. Beginners always neglect the
large relations of light and shade and colour, dwelling on
those that are small ; whereas the aim of the true artist is
the production of a broad general effect by the use of a
few masses of colour, properly interchanged and contrasted,
variety being gained not so much by the introduction of
new colours as by the repetition of the main chords. Va-
rious modes of contending with this evil have been sug-
gested. One of the simplest is making the colour-sketches
so small that there is hardly room for anything but the
main masses of colour, the use of small brushes meanwhile
being avoided. Corresponding to this, it is frequently
found that if a picture by a beginner is actually cut up
into two or more parts, the fragments thus produced are
better in the matter of chromatic composition than the ori-
ginal.

There are several other stumbling-blocks that are en-,
countered with much regularity by those who make their
first essays in colour. One of the most important is the
tendency to employ in the painting colours that are vastly
more intense than those displayed by nature. The colours
of nature are usually pale and low in intensity, even when
they make upon the beholder just the reverse impression ;
and a practical knowledge of this fact is not to be imme-
diately attained. Distant fields, for instance, often appear
to be of a rather intense green. hue, when the colour actu-
ally presented to the eye may be scarcely more than a grey
having in it a faint tinge of green. The actual colours

320 MODERN CHROMATICS.

exhibited by different parts of a landscape may be advan-
tageously studied by isolating them, according to a sugges-
tion of Ruskin, with the aid of a small aperture, half an
inch square, in a piece of white cardboard, held at arm's
length. By this simple proceeding the student can con-
vince himself of the true nature of the tints composing a
scene, for when thus isolated they are not heightened by
contrast. With such square patches of colour, the judg-
ment is not so much afPeoted by the memory of the hues
which the objects exhibit at short distances, or by what
artists call their local colour. The local colour of grass is
green, but if placed at a distance it may display a great
variety of pal^ tints, scarcely even greenish ; yet owing to
the action of the memory the distant grass stUl suggests,
not the idea of a variety of pale delicate • greys, but of its
local colour, green. The illustration is very old, but the
principle applies not only to the greens, but to all colours :
all will be altered by distance, by the brightness of the il-
lumination or by its colour. The hues of all objects are
also greatly affected by their surroundings, as explained in
the chapter on contrast ; and this is another source of per-
plexity and confusion to the beginner, who is constantly led
astray by appearances due to this cause. The extent of the
difficulty can be appreciated when we remember that con-
•trast affects not only the intensity of the colour, but its
position in the chromatic circle, and also its apparent lumi-
nosity, and is particularly lively in the case of the pale
colours of nature. It is as well to meet this difficulty fair-
ly face to face, and, instead of spending all the disposable
time in endeavoring to solve the riddles of contrast pre-
sented by nature, to reverse the process, and occasionally to
construct in the studio simple chromatic compositions found-
ed on the known laws of contrast, and thus study its effects
by experiment as well as by observation.

The appearance of colour, as has been explained in an-
other chapter, depends also greatly on gradation ; colour

COLOUR IN PAINTING AND DECORATION. 321

which is uniform appearing hard and disagreeable, while
the same tint when gently varied becomes pleasing as well
as truer to nature. Gradation of colour is almost universal
in nature, and a considerable part of the education of the
student consists in its study and practice. The uneducated
eye feels the effect of gradation in nature and in a painting,
but is quite unable to trace the delicate play of colour and
light and shade on which it depends. Skill in the use of
gradation gives the artist great power to manage large
masses of nearly uniform colour, and an astonishing mas-
tery over colour-combinations which inherently are of
doubtful value.

The enormous influence of good, decided drawing has
already been alluded to, but we return again to the matter
for a moment, to insist on the added lustre which all the
tints of a painting acquire when connected with good, well
wrought-out light-and-shade effect. The beginner can most
easily convince himself of the great influence which the
light-and-shade effect exercises on the colour, by copying
the engraving of some simple subject by a master in such
, colours as may seem most appropriate, and then comparing
the coloring thus obtained with that of his own original de-
signs. The selection of pigments in both cases is by the
same hand, but it will be found that the masterly light and
shade has given a value even to the colours, which without
this little plagiarism they would not have possessed.

In painting, the selection of subjects on account of their
chromatic qualities is a very important element of success.
If is only by experience that artists gradually learn better
and better how to select their subjects, and the mistakes of
beginners' in this respect are often a source of prolonged
discouragement. Subjects which contain much green are
invariably difficult to manage, and should as far as possible
be avoided in the earlier stages ; green fields, green trees,
green mountains, all need great skill if the colour is ren-
dered with any approach to fullness. This is the reason

323 MODEEN CHROMATICS.

that the older landscapists lowered the colour of their trees
to a dull olive-green, and even to brown. Combinations of
green, blue, and grey or white are very common in nature,
but difficult to handle, and necessitate the use of an un-
usual amount of gradation. ' From a chromatic point of
view the combination is poor ; as seen in nature, we do not
value it less on that account, perhaps more. It is delight-
ful to see how much can be accomplished with elements of
such doubtful value. Again, effects which are much de-
pendent for beauty on very high degrees of luminosity are
difficult, for their pale colours, when transferred to canvas
and robbed of their natural luminosity, are apt to appear
tame enough. In the same category we must place dis-
tances, with their excessively pale, delicate tints. The col-
ouring in nature seems very brilliant, but in point of fact
the effect is produced partly by mere luminosity, and partly
by the aid of tints differing so slightly from each other and
from grey that the problem of imitation, either literal or
free, is not at all easy.

We might go on in this way adding to the catalogue of
art difficulties, but perhaps it will be asked. What subjects ,
are easy ? The truthful answer is simply, that all are diffi-
cult if even a moderate degree of excellence is demanded.
The painter who wishes to excel in colour devotes his life
to this object, and is constantly accumulating studies and
sketches from nature of all kinds of subjects, some quite
elaborate, others with less detail, many mere colour-notes.
Quite often beautiful effects of colour in nature last only a
few minutes ; these will be treasured in the memory and
transferred to paper or canvas the next day, the sketch
being completed only far enough to fix the facts in the
memory of the artist. Many, experimental sketches will be
made, not directly from nature, but with a view, as it were,
of guessing at the elements on which certain difficult or
evanescent chromatic effects depend, and also for the pur-
pose of ascertaining their relations to mere light and shade.

COLOUR IN PAINTING AND DECOKATION. 323

This varied work in colour will be accompanied by constant
practice in black and white for light and shade, and in out-
line for form, since bad drawing is ruinous to colour. All
the while there will be more or less anxious study of the
works of good colourists, ancient and modem ; and this work
will be pushed on month after month with patient energy,
till, after a score of years or so, the student finally, if gifted,
blossoms out into a colourist.

NOTE ON TWO RECENT THEORIES OF COLOUR.

Herino has lately proposed a theory of colour which la quite different
from that of Young. According to the new theory, the retma Is proyided
with three visual substances, and the fundamental sensations are not three

but six

Black and "White.
Red and Green.
Blue and Yellow.

Each of these three pairs corresponds to an assimilation or diassimilation
process in one of the visual substances ; thus red light acts on the red-
green substance in exactly the opposite way from green light, and when
both kinds of light are present in suitable proportions a balance is eflfected,
and both sensations, red and green, vanish.

Furthermore, according to this theory, all the colours of the spectrum
also affect the black and white substance jn the same way that white light
does ; for example, red light affects the red-green substance and produces
the sensation of red, but it also acts on the white-black substance, and the
sensation of red is mingled with that of white — to a large degree. Conse-
quently, according to this theory, the white which is produced by mix-
tures of red and green light ought to have a less intensity than the sum of
the separate components ; but according to the experiments of the author this
is not the case.* For further details the reader is referred to the original
paper, " Lehre vom Lichtsinne," Vienna, 1878.

In ISTB F. Boll discovered that the retina contained a red or purple
substance that quickly disappeared on exposure to light. Boll and Kiihne
have both studied the effect of monochromatic light on this coloured sub-
stance, and it was found that red light intensified the hue at first and after-
ward caused it to fade slowly. The action of yellow light was slow ; green,
blue, and violet light acted more quickly. On observations of this charac-

* " American Journal of Science and Arts," October, 1877.

NOTE ON TWO RECENT THEORIES OF COLOUR. 325

ter Kiilme has constructed a theory of viBion. He supposes that the waves
of light give rise iu the retina to different compounds according to their
length, and thus produce the different colour-sensations. If three such
compounds are thus produced, giving rise to the sensations red, green, and
violet, then this new theory is identical with that of Young ; if there are
five such compounds, furnishing the sensations red, yellow, green, blue, vio-
let, then the apparatus for yellow and blue has been duplicated in the
retina, since it can be shown that a mixture of the sensations red and green
gives that of yellow, a mixture of green and violet that of blue. Good rea-
sons can also be adduced to render probable the idea that yellow and blue
are not fundamental sensations, but mixtures (compare the observations of
Bezold in Chapter XII.). For additional information the reader is referred
to the papers of Kuhne published in the " Verhandlungen des Naturhisto-
rische-medicinischen Vereins zu Heidelberg, ISII-'Id."

Ilif D E X,

Abnormal perception of colour, 92.

Absorption, production of colour by, 65.

Absorption and Irue mixture of light com-
pared, 143.

Absorption of light by stained glass, 65,

AgasBiz, A., observations of on colour of
flounders, 101.

Airy opposed to Brewster's theory, 109.

Albert on colour-photography, 87.

Alharabra, decoration of, 312.

All pigments reflect some white light, 76.

Antique glass, colours of, 52.

Aubert on mixtures of blue and white, 196 ;
relative luminosity of white and black
paper, 188 : sensitiveness of eye to mix-
tures of white and coloured light, 89.

Bert, observations of on chameleon, 101 ;

on crustaceans, 10.
Bezold, prismatic colours change with their

brightness, 181.
Bierstadt, experiments of in colour-photog-
raphy, 87.
Blake, Eli, his mode of recomposing white

light, 29.
Blue, complement of, 177.
Bokowa, Maria, artiflcial colour-blindness

of, 97.
Brewster, Sir David,'colour theory of, 108;

discoverer of cross and rings, 47.
Briicke, his apparatus for complementary

coloiU'S, 161 ; observations on mixtures

of blue and white, 196.
Brucke''s solution, imitates sky tints, 154.
Bunseu's experiment on colour of water,

81.

Calculation of number of visible tints, 40.
Campbell, J., experiments on photographing

colours by, 86.
Chameleon, its power of imitating colours,

Charts, colour, 213, 220.
Chevreul, colour-chart of, 222.
Chrome-yellow, spectrum 0% 76.

Colour, abnormal perception ot, 92; appar-
ent spreading ot, 284; balance of, 801, 803 ;
by moonlight, 187 ; change o^ with wave-
length, 17, 27 : changed by illtmainationt
181 ; effect of jamp-light on, 154; grada-
tion of, 276 ; has more than one comple-
ment, 172 ; how affected by mingling it
with white, 194 ; is subjective, 17 ; less
important than form, 806 ; musical theo-
ries oii 303 ; of vegetation, 82 ; of water,
61; produced by absorption, 65; pro-
duced by dispersion, 17; produced by
electric current, 9 ; produced by opales-
cent media, 58 ; production of by inter-
ference, 50 ; relative luminosity of depend-
ent on degree of illumination, 189 ; repro-
duction of by photography, 86 ; sensation
of, produced by white light, 92 ; value o^
from practical point of view, 805.

Colour and wave-length do not change
equally, 27.

Colour-blindness, 95, 96; of artists, 100;
means of -helping, 98; to green, 98 ; to
red, 96.

Colour-chart, Chevreul's, 222 ; of Du Fay,
222 ; of Le Blond, 222.

Colour-charts, 213, 220.

Colour-combinations, bad, 292; bad owing
to absence of warm colours, 298; bad
ovring to intensity, 298 ; pairs, 286-299.

Colour-cone, 216; and cyUnder, impossible
to execute, 217.

Colour-contrast, 235-273.

Colour-cylinders, 215.

Colour-diagram, Maxwell's, 224; Eood''B,
233.-

Colour-equations, 184.

Colour-sensations that are not fundamental,
115.

Colour-tbeoiy of Brewster, 108 ; of Toung,
118 ; of Young and Helmholtz, 113.

Colour-triangle, 221.

Coloured li^t when bright becomes more
yellowish, 181-183.

Coloured photographs, indirect process, 87.

Coloured silk and wool compared, 79.

Colours can be too pure and intense, 80;
combined in triads, 299-801 ; in combina-
tion, mode of studying, 290; fandamental,
defined, 120 ; in mixture represented by

INDEX.

327

weightfl, 318; mixture of by binocular
vision, 158 ; mixture of by Lambert's ap-
paratus, 189; mixture of on retina of ob-
server, 279; of metals, 84; of ordinary ob-
jects due to absorption, 65 ; of pigments
due to absorption, 65 ; of prismatic spec-
trum, 18 ; of woven fabrics due to ab-
sorption, 78; photometric, comparisons of
not absolute, 190 ; prismatic, change due
to brightness, 181.

Colours, complementary, 161 ; explained by
Young's theory, 176 ; by gas-light, 173 ;
in combination, 294; of polarized light
rather pale, 177.

Complementary colours, by gas-light, 173 ;
explained by Young's theory, 176 ; meth-
od of studying with Maxwell's disks, 167 ;
luminosity of^ 164: no fixed relation be-
tween their wave-lengths, 175 ; of polar-
ized light are rather pate, 177 ; table of, 168.

Constants of colour, 30-210.

Contours, 811.

Contrast, 285-273; experiment with shad-
ows, 254 ; hurtful, 297 ; intensity of col-
ours being different, 263 ; of black, white,
and grey, 267 ; of black, white, and grey
with colours, 270 ; of pale and dark col-
ours, 258-268 ; - simultaneous, 241-245 ;
strength with different colours, 26J ; suc-
cessive, 285-242; table of eflfects of, 245.

Contrast-circles, 248.

Contrast-diagram, 250.

Cross and rings produced by polarized light,
47.

Cross, C, experiments of in colour-photog-
raphy, 87.

Carves for action of red, green, and violet
on the eye, 198.

Dalton, colour-bhndness of, 97.

Dalton's eye-piece, 36.

B'Arcy on duration of impression on ratina,
203.

Dark lines of spectrum, 20.

Decoration, different kinds of, 309 ; fh.lse aim
in, 807, 318 ; use of one colour in, 808, 810 ;
use of several colours, 809, 811.

Decoration and painting divergent in aim,
806.

De Haldat on binocular perception of colour,
159.

DichroosCope, Dove's, 187.

Difiraotion grating, 23 ; Eutherfurd's, z&.

Diffraction spectrum, 23.

Disks, complementary, 170 ; MaxwelVs,
109 ; rotating, used in the study of Young's
theory, 180.

Dispersion, production of colour by, 17.

Dove on binocular perception of colour, 159 ;
his comparison of effects of absorption and
true mixture of light, 143 ; dichrooscope
of, 137 ; his method of studying comple-
mentary colours, 165; observations on
relative luminosity of red and blue, 189;
photometric experiments on revolving
disks, 206 ; theory of lustre, 280.

Draper, H., opposed to Brewster s theory,
109.

Drawing, importance of 817, 321.

Du Fay, colour-chart of, 222.

Duration of impression on retina, 202; in

case of animals in motion, 203 ; in case of

ocean waves, 207.

Electricity, production of colour by, 95.
Emerald -green, spectrum of, 75.
Erythroscope, 83.
Etchings, blending of white and black on

retina of observer, 282.
Eye, colour of, 58 ; more sensitive to change

of wave-length in certain regions of the

spectrum, 27.

Favre, examination of colour-blind persons
by, 99.

Feathers, colour of, 50.

Fechner on colours of after-images, 93.

Field, chromatic equivalents of, 801 ; experi-
ments on pigments by, 88, 89.

Fixed hues of solar spectrum, 20.

Fluorescence, production of colour by, 62.

Foucault on binocular perception of colour,
159.

Fraunhofer, discovery of fixed linos by, 20.

Fundamental colours defined, 120 ; intensity
oi^ in prismatic spectrum, 123; Wiinsch
on, 123.

Fundamental colour-sensations, how deter-
mined, 115.

Gas-light, effects of, on colours, 154.
Gibbs, Wolcott, on duration of impression

of prismatic colours, 206,
Glass, opalescent, 55.
Glass under strain, colour of by polarized

light, 48.
Gold used in painting, 85.
Gradation of colour, 276; rapid, often un-

pleasing, 275 ; subordinate in decoration.

Green in colour- combinations, 295.
Grunow, William, spectrometer of, 21,

Harris, colour-blindness of, 99.

Helmholtz on colour-blindness, 97; coloup-
• blind zone of normal eye, 97 ; colours of
after-images, 93 ; experiment of with blue
and yellow glass, 18S; on fundamental
colours, 120 : mixtures of blue and yellow,
190 ; mixture of prismatic colours, 111,
126; no fixed relation exists between
wave-lengths of complementary colours,
175 : prismatic colours change with their
brightness, 181.

Helmholtz Mid Young, colour theory of,
113.

Helmholtz's spiral disk for after-images, 98.

Bering's theory of colour, 324.

Holmgren, examination of colour-blind per-
sons by, 99.

328

INDEX.

Fuddart, remarkable case of colour-bUnd-

ness, 99.
Hue, 36.

Illumination, monochromatic, 102.

Indigo, complement o^ 178; its real colour,
21 ; term improperly used by Newton,
ii>. ; unfit designation of spectral hue, ib.

Insects, colour o£ 51,

Interval, small, SJ78.

Kiihne's theory of colour, 824.

Lambert, apparatus of for mixing coloured
light, 110 ; his apparatus used for mixing
colours, 139 ; colour-chart of, 222.

Lamp-light, effects of on colours, 154.

Le Blond, colour-chart of, 222.

Listing's experiments on spectrum, 26.

Luminosity of colour, 3B.

•Lustre, Dove's theory ot, 280.

Magnus, Hugo, on development of sense for
colour, 101,

Maxwell on colour-blindness, 108; funda-
mental colours, 120 ; intensity of ftmda-
mental colours in prismatic spectrum,
128 ; mixture of prismatic colours, 126

Maxwell's colour-diagram, 224 ; reconstruct-
ed, 228.

Maxwell's disks, 109, 130-

Mayer, A. M., Ms history of Toung's theory,
123.

Mayer, H., hie arrangements for contrast,
- 260.

Mayer, T., co'our-chart of, 222.

Medium with which pigments are mixed,
77.

Melloni opposed to Brewster's theory, 109.

Metals used in painting, 85.

Mile, his mode of mixing colours, 139.

Milk, colours produced by, 53.

Mixture of blue and yellow light makes
white, 112 : of colours by binocular vision,
158; of coloured rays of prismatic spec-
trum, 126; of different-coloured light,
124 ; of pigments, theory and effects, 141 ;
of prismatic colours. 111 ; of white and
coloured light. 31, 32.

Monochromatic Illumination, 103.

Monoehromy, 808, 310.

Moonlight, colour of, 187.

Morton, H., thallene described by, 63.

Mountains, distant, colours of, 59.

Miiller, J. J., on randamental green, 121 ;
green light in mixture produces a whitish
tint, 119: mixture of prismatic colours,
126.

Newtotfs diagram for the colour-blind, 105;

for lamp-light, 105.
Newton's experiment, IB.

Niepce de Saint-Victor's experiments on

photographing colours, 86.
Nitrate of potash, colours oi^ in polarized

hght, 47.
Nobert, difiraction grating of, 23.
Normal spectrum, 24, 25; appearance oi,

122.

Painting, first practice, 318.

Painting and decoration divergent in aim,
806.

Painting Mid drawing, connecting links, 314.

Pettenkofer's process, 58.

Pfaff, experiment of on optic nerve with
dectricity, 9.

Phosphorescence, colours of, 64.

Photographs, instantaneous, peculiarity of;
207.

Photography, coloured, thus fer a failure, 86.

Pierce, Charles, darkened red becomes more
purphsh, 185; flmdamental green, 120;
observation on colour-blindness, 96 ; pho-
tometric researches of, 41,

Pigments, action of light on, 88; appear-
ance of affected by medium, 77 ; compara-
tive luminosity of, 75 ; only three abso-
lutely essential, 108 ; peculi^ properties
of influence their mixtures, 124 ; used for
set of complementary disks, 179,

Pigments and stained glass compared, 78.

Pisko, F. J., on fluorescence, 68.

Plateau on duration of impression of .col-
oured light on retina, 206; photometric
experiment of, 305,

Platino-cyanide of barium used for fluores-
cence, 63.

Polarization, production of colour by, 48.

Polarizing apparatus, simple, 44.

Polychromy, 811.

Pouchet, observations of on colour ol floun-
ders, 101.

Prayer on colour-blindness, 97, 98.

Prismatic colours, mode of isolating, 19.

Prismatic spectrum, 18.

Purity of colour, 82.

Purkinje, relative luminosity of warm and
cold colours dependent on the degree of
illumination, 189.

Purple, how produced, 28.

Eagona Scina. apparatus of for contrast, 257.

JRecomposition of white light, 28.

Eed, sensation of, more intense when green
and violet nerves are fatigued, 118.

Red light, action of on green nerves, 117. ,

Eeflection, by polished surfaces, 11 ; by
rough surfaces, 13 ; by water, 12 ; of col-
oured light by rough surfaces, 18 ; of light,

Eegnault on binocular perception of colour,
159.

Retina, 10.

Eood on binocular perception of coloui^
159 ; colour-blindness produced by a shock
to nervous system, 95 ; coloured spaces
in spectrum, 23, 24; colours corresponding

INDEX.

329

to certain wave-lengths, 26; comparison
of eflfectB of absorption and true mixture
of light, 146 ; complementary colours by
lamp-light, 178; complementary disks,
171: complement of red, 164; contrast-
circles, 248: contrast-diagram, 250; esti-
mation of the coloured spaces in the pris-
matic spectrum, 22 ; effects of gas-light
on colour, 156 ; experiments on darkened
pigments, 185, 188 ; experiments on pig-
ments, 90 ; expei-iments on subjective
colours, 94 ; grey has a tendency to blue,
191; luminosity of pigments, 85; method
of comparing the luminosity of white and
coloured sumces, 84 ; mixtures of white
and coloured light, 197: peculiarity of
thin layers of pigments, 199 ; position of
pigments In normal spectrum, 38 ; quan-
titative analysis of white hght, 41 ; reflect-
ing power of black disks, 134 ; reflection
of coloured light from coloured surfaces,
149; size of coloured spaces in normal
spectrum, 24; saturation-diagram, 283;
time necessary for perception of colour,
102 ; wave-length corresponding to differ-
ent colours, 26,

Eutherfturd, observations of on blue of the
spectrum, 121 ; prismatic colours change
with their brightness, 181.

fiutherfurd^s automatic spectroscope, 8T ;
difitaction grating, 28, 26 ; diffi-action
plates, 88.

Buskin on mixing colours, 140 ; on colour-
gradation, 278.

Santonin, colour-blindness produced by, 95.

Saturation-diagram, Rood^e, 288.

Schelske on colour-blind zone of normal
eye, 97.

Beeoeck's observations on colour-blindness,
96.

Seguin on colours of after-images, 93.

Belenlte, colours of in polarized Hght, 44.

Shadow conftjspd with reflection, 15.

Sfaells, colours of, 51.

Simler's erythroscope, 88.

Sky, colours of, 58.

Small intervals, table of, 274.

Smoke, opalescent colours of, 65.

Solar spectrum, 18.

Soap-bubbles, colours of; 49 ; finely pamt-
ed, 46.

Spectra due to chloride of chromium, 72.

Spectrometer, 21.

Spectroscope, 20.

Spectnim, ditfraction, 28.

Spectrum due to blue glass, 70 ; to green
glass, ib. ; to green leaves, 82 ; to red
glass, 66 : to smalt paper, 75 ; to stained
glass varies with thickness of glass, 71.

Spectrum, normal, 28 ; appearance of, 122 :
normal and prismatic compared, 28; of
orange glass, 69 ; prismatic, 18.

Stained glass, colour transmitted by, 16 ;

and pigments compared, 78.
Stokes, researches of on fluorescence, G2.
Successive contrast, 285-242.
Sugar, colours of in polarized light, 46.
Sulphide of barium, phosphorescence o^ 64;

of calcium, ib. ; of strontium, ib.
Sunset colours, normal series, 61.

Table of fixed Unes in solar spectrum calcu-
lated to 1,000 parts, 22.
Tables of colours in pairs, 286-291.
Tait, colour-blindness produced by fever, 95.
Tartaric acid, colours of in polarized hght, 46.
Thallene, fluorescence of, 68.
Theories of colour, recent, 824.
Thin flhns, colours of, 49.
Translucency, 15.
Transmission of light, 15,

Uranium glass, for production of fluores-
cence, 62.

Vegetation reflects red light, 88.

Veins, colours of, 58.

Velvet, colours of, 79.

Vermilion, specb-um of, 76.

Vierordt's photometric researches on the

spectrum, 83.
Vision, theory of; 11.
Von Bezold, observations of on darkened

prismatic spectrum, 183.

Warm and cold colours, proportion of in

white light, 42.
TVave-Iength made greater In fluorescence,

62.
Wave-length and colour do not change

equally, 27.
Waves of hght produce sensation of colour,

17.
White lead on dark ground appears bluish,

56.
White light reflected from surfiicea of pig-
ments, 76.
Window-glass, old, colours of, 52.
Woinow on colour-blind zone in normal eye,

99.
Wollaston noticed fixed lines, 20.
Wiinsch on fundamental colours, 122.

Yellow, complement of, 177 ; ftom a mix-
ture of red and green light, not very bril-
liant, 116.

Young, colour theory ofi 118.

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Telegraphy. By W. H. Peeeoe, C. E.,

and ji SivEWRiGHT, M. A. Cloth, $1.50.
Mailway Atmliamxs. ■ By J. W. Baeey,

C.E. Cloth, $1.50.
The Art of Mlectro-Metatturgy. By G.

Gore, LL.D., P.E.B. Cloth, $2.60.
Introduction to the Study tf Chemical

Pliilosophy. By W. A. Tildes, D. So.

Lond., P. 0. S. Cloth, $1.50.
The Elements of Machine Design. By

Professor W. C. Unwih, C. E. Cloth,

$1.60.
Treatise on Photography. By Captain

W. De "Wive LESLIE Abhey, P. U. S.

Cloth, $1.50.

(Otker volumes in preparation.)

" This admirable series of text-books is invaluable for the use for which it was origi-
nally planned. Several of the authors are preeminent in their own specialty, and their
works must have been of immense service to the numerous class of students for whom
they are chiefly intended. Taking the series as a .whole, it would be a difficult task to
single out another Ust of text-books on the same or collateral subjects in our language
which could be compared with them, either in regard to quality and price, or that
are so well fitted for the instruction of engbieering students, or for students generally
In our public and science schools." — London Examiner.

D. APPIEION & CO., Publishers, 549 and 551 Broadway, New York.

TEE EXPERIMENTAL SCIENCE SERIES.

LIGHT:

A Series of Simple, Entertaining, and Inexpensive Experiments in the
Phenomena of Light, for the Use of Students of Every Age.

BY ALFRED M. MAYER and CHARLES BARNARD.

Neat 12Mo tolume, fully illustrated. . . Cloth, price, |1.00.

" Professor Mayer has invented a series of experiments in Light wliich are
described by Mr. Barnard. Nothing is more neoeaeary for sound-teaching than
experiments made by the pnpil, and this book, by considering the difficulty of
costly apparatus, has rendered an important service to teacher and student alike.
It deals with the sources of light, reflection, refraction, and decomposition of
light. The experiments are extremely simple and well suited to young people."
— Westminster Seview.

"This work describes, in simple language, a number of experiments illus-
trating the principal properties of light, by means of a beam of sunlight admitted
into a dark room, .and various contrivances. The experiments are highly in-
genious, and the young student can not fail to learn a great deal from the book.
As an example of the eflfective experimental method employed, we may specially
mention the device for illustrating the refraction of light. This book is specially
designed ' to give to every teacher and scholar the knowledge of the art of experi-
menting.' "—T/ie Quarterly Journal Qf Science (London).

"A singularly excellent little hand-book for the use of teachers, parents, and
children. The book is admirable both in design and execution. The experi-
ments for which it provides are so simple that an intelligent boy or girl can
easily make them, and so beautiful and interesting that even the youngest chil-
dren must enjoy the exhibition. The experiments here described are abundantly
worth all that they, cost in money and time in any family where there are boys
and girls to be entertained."— iV«w York jEvening Post.

" The experiments are capitally selected, and equally as well described. The
book is conspicuously free from the multiplicity of confusing directions with
which works of the kind too often abound. There is an abundance of excellent
Illustrations."— iVsw York Scientyic American.

" The experiments are for the most part new, and have the merit of com-
bining precision in the methods with extreme simplicity and elegance of design.
The value of the book is further enhanced by the numerous carefully-drawn cuts,
which add greatly to its he&atj."— American Journal qf Science and Arts.

D. AFPLETON & CO., 549 & 5S1 Bkoadwat, New York.

THE EXPEBIMEJfTAL SCIEJVCE SERIES.

SOUND:

A Series of Simple, Entertaining, and Inexpensive Experiments in the
Phenomena of Sound, for the Use of Students of Every Age.

BY ALFRED MARSHALL MAYER,

Professor of Physics in the Stevens Institute of Technology; Member of the
National Academy of Sciences, etc.

Uniform with "LIGHT," first volume of the Series.

Neat 12mo volume, fdlly illustrited. . . Cloth, price, $1.00.

"The object of the volume is to present the leading phenomena of Sound in a
simple and entertaiuing manner, by the use of such materials as are almost every-
where at hand, and with apparatus which any ingenious ttudent can construct
for himself. To present the elements of an abstruse subject in such a way as to
make the exposition easily comprehensible by a mind not specially trained lu
it, and at the same time correct and satisfactory from a scientific point of view,
is one of the most difficult undertakings in the v/ork of an instructor. Add to
this the task of bringing the experimental illustration of a science like that of
acoustics, which requires such refinement in the apparatus and its manipulation,
within the resources of every one, and we have the difficulty very greatly in-
creased. Professor Mayer's well-lmown experimental skill has enabled him to
accomplish the work in an admirable manner, and be has laid under obligation
to him not only the student and the amateur experimenter, but the teacher, who
will derive many valuable suggestions as to hie own work from this little volume.
The subject is arranged in a very clear and methodical manner, and treated in a
vivacious and entertaining style. The experiments, many of which are novel,
unite extreme simplicity with elegance of conception and scientific precision,
and can not fail to interest and stimulate the minds of the students into whose
hands the volume may fall. The illustrations, which are Bumeroua, are ex-
cellently done, and give the book a very attractive appearance."— ^mericon J&ar-
nal (if Science and Arts.

" It would really be difficult to exaggerate the merit, in the sense of consum-
mate adaptation to its modest end, of this little treatise on ' Sound.' It teaches
the youthful student how to make experiments for himself, without the help of
a trained operator, and at very little expense. These hand-books of Professor
Mayer should be in the hands of every teacher of the young." — 2^€W York Sun.

" An admirably clear and interesting collection of experiments, described with
just the right amount o>abstract information and no more, and placed in pro-
gressive order. The recent inventions of the phonograph and microphone fend
an extraordinary interest to this whole field of experiment, which makes Pro-
fessor Mayer's manual especially opportune." — Boston Courier.

D. APPLETON & CO., 549 & 551 Broadway, New York.

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